Ford: High altitude requires 0W oil. Why?

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Think about the pickup tube issue, and how it approximates how a mercury barometer works, which after all is an instrument used to measure atmospheric pressure. Forget the pressure side of the pump for the moment. Then re-read my theory and some of @ZeeOSix 's explanations. Maybe it will make sense then.
I have, believe me. A mercury barometer has a sealed chamber, so it operates based on a net pressure differential between that chamber and ambient pressure. The engine's oil sump and galleys (input and output sides) are both exposed to ambient pressure. They may be isolated from each other, but neither is isolated from ambient atmospheric pressure.
 

Alien

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I have, believe me. A mercury barometer has a sealed chamber, so it operates based on a net pressure differential between that chamber and ambient pressure. The engine's oil sump and galleys (input and output sides) are both exposed to ambient pressure. They may be isolated from each other, but neither is isolated from ambient atmospheric pressure.
No: as soon as the pump starts running, it creates a partial vacuum in the pickup tube, similar to the vacuum in the top of a mercury barometer. So now you have a net pressure differential which lifts the oil. The inside of the pickup tube is not directly exposed to atmospheric pressure once the pump is running, only via the oil in the pan.
 
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ZeeOSix

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PD pumps need a minimum absolute inlet pressure (called "Net Positive Suction Head Required", aka NPSHR) to ensure the fluid entering the pump doesn't start to cavitate which could impact pumping performance.


As an example for talking purposes, look at Figure 5 (marked up copy below). This information definitely says that PD pumps are sensitive and impacted by the absolute pressure on the fluid flowing into the inlet (suction/vacuum) side of the pump. The absolute pressure on the fluid being pushed into the inlet side of the pump is equal to the head pressure (from gravity) + the atmospheric pressure pushing down on the fluid. The ATM factor is much stronger than the head factor since the depth of oil in an engine sump really isn't very deep (probably only 8-10 inches).

Oil sitting in an engine sump has an absolute pressure (PSIA) equal to the head pressure (based on the depth of the pickup tube below the oils surface) + the absolute ATM pressure pushing down on the surface of the oil. So at high elevation, as the ATM pressure decreases so does the absolute pressure pushing down on the oil at the pump pickup. This is going to impact the pumping performance of the PD pump. This factor might also be a factor when it comes to the SAE J300 "Pumpability Viscosity" specification. But I'm thinking the SAE J300 spec is based on pumpability only at ATM pressure, and doesn't consider high elevation like Ford seems to have addressed. Seems Ford wants owners to use a lower W rating under certain specified conditions (higher elevation of 7500+ ft with cold temperatures of -20C/-4F or colder) to ensure the pump is performing properly during cold start-up.

You can see in the Figure that if the absolute inlet pressure is decreased the fluid viscosity would have to also decrease in order to keep the pump happy. This also depends on the exact pump design and RPM, but all PD pumps will have a performance graph like Figure 5. I added the red and green dots at two different pump RPM at the PSIA that corresponds with sea-level and 7500 ft elevation for talking purposes. If you pick any pump RPM, you can see that as the inlet absolute pressure decreases (ie, you go from sea-level to 7500 ft), the viscosity of the fluid would have to decrease in order to ensure proper inlet performance of the PD pump.

1663357908022.jpeg
 
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ZeeOSix

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Yes, that's the theory. However, the part I bold faced doesn't seem right. It's gravity, not atmospheric pressure, that causes the oil in the pan to fill in and replace the oil sucked into the oil pump intake. It can't be atmospheric pressure, because that pressure is ambient -- there is no atmospheric pressure differential across the intake & output sides of the oil pump.

I emphasize atmospheric because of course, the pump itself creates a pressure differential across the intake & output sides! But atmospheric pressure does not contribute to this because it's ambient - equal on all sides.
The atmospheric pressure (14.7 PSIA at sea-level) pushing down on the oil in the sump certainly does contribute to the force that helps push the oil into the pump pick-up tube and inlet side of the pump. Oil/fluids will flow from high to low pressure, and when the pick-up tube and inlet side of the pump is below the total pressure on the oil, then the oil will flow into the pump. The larger that delta-p between the gravity + ATM pressure on the oil and the pump inlet (suction/vacuum), the more driving force there will be on moving the oil from high to low pressure. Given a specific PD pump, it will produce a certain level of suction (vacuum) on the inlet side based on it's exact design and the speed it's turning ... regardless of what's going on at the output side. That's why a non-pressure regulated PD pump can maintain the same suction level at a constant RPM and literally produce 100s of PSI on the outlet side (given enough power input of course) depending on the resistance to flow connected to the outlet.
 
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I don't doubt the chart. Yet I want to know "why". Here's what doesn't seem right. Atmospheric pressure doesn't only push "down" it pushes outward in all directions.

Now imagine a bucket full of a fluid like water or oil.

If atmospheric pressure pushed "down" on the fluid then it would weigh more at high pressure and less at low pressure. But atmospheric pressure doesn't affect its weight. It weighs the same as ambient pressure rises and falls.

Now poke a hole in the side of the bucket near the bottom. The rate at which the fluid drains through the hole does not depend on ambient atmospheric pressure. Ambient pressure pushes down on the fluid in the bucket, and also pushes in from outside the hole, equally. No net effect or impact on the rate of drain. It drains at the same rate regardless of ambient air pressure.

Now say the bucket is an engine oil sump and connect the hole to a PD oil pump. If lower air pressure didn't slow down the drain rate through the hole, how can it affect the intake of the pump?
 

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I don't doubt the chart. Yet I want to know "why". Here's what doesn't seem right. Atmospheric pressure doesn't only push "down" it pushes outward in all directions.

Now imagine a bucket full of a fluid like water or oil.

If atmospheric pressure pushed "down" on the fluid then it would weigh more at high pressure and less at low pressure. But atmospheric pressure doesn't affect its weight. It weighs the same as ambient pressure rises and falls.

Now poke a hole in the side of the bucket near the bottom. The rate at which the fluid drains through the hole does not depend on ambient atmospheric pressure. Ambient pressure pushes down on the fluid in the bucket, and also pushes in from outside the hole, equally. No net effect or impact on the rate of drain. It drains at the same rate regardless of ambient air pressure.

Now say the bucket is an engine oil sump and connect the hole to a PD oil pump. If lower air pressure didn't slow down the drain rate through the hole, how can it affect the intake of the pump?
I give up. You are completely confused. I tried to explain, but I apparently can't help you in your confusion.

You cannot expect to get an education in physics on an internet discussion board.
 
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I give up. You are completely confused. I tried to explain, but I apparently can't help you in your confusion.
You cannot expect to get an education in physics on an internet discussion board.
The problem is, having university physics and a degree in math, I am over-thinking this. Otherwise I'd be satisfied with the intuitive notion that ambient air pressure "pushes down" on the fluid. Of course that seems obvious. But if it were the true explanation, then why doesn't a bucket with a hole near the bottom drain slower at high altitude?

So while I accept the conclusion that ambient air pressure does indeed affect the pump, and this does explain the original question of why Ford recommends a lower viscosity oil based on altitude/pressure not just temperature, I'll continue to noodle on this to see if I can convince myself of the true reason WHY this is the case. I'm sure it's something simple that I'm overlooking.
 

ZeeOSix

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I don't doubt the chart. Yet I want to know "why". Here's what doesn't seem right. Atmospheric pressure doesn't only push "down" it pushes outward in all directions.
You have to look at what forces are on the oil in the sump that will move the oil to the pump inlet when the pump is turning and the inlet is at a lower pressure (suction/vacuum) than the pressure on the oil at the pick-up. Fluid always moves from high to low pressure (delta-p). The pump inlet chamber will be at some level of vacuum depending on the exact pump design, health and its rotation speed ... and that holds true regardless of what the pump outlet is doing - it's the beauty of a PD pump. On an ideal PD pump, the inlet side will not be effected by the outlet. A PD pump that is producing 100s of PSI on the outlet (depending on what it's connected to) will still produce the same essential vacuum/suction on the inlet. The force on the oil in the sump is the only thing that moves the oil into the pump pick-up tube and into the pump. Imagine if you could pressurize just the oil sump volume (but not the rest of the engine's volume) to 65 PSI of air pressure (50 PSI above ATM). Pressurizing the sump would help push that oil more effectively through the pick-up tube to the inlet of the pump which would still be at the same vacuum level even though the outlet is at high pressure. If the oil became thick enough, and/or the driving force of the oil low enough (near zero air pressure for example), then the flow going into the pump will become less than what the pump is capable of moving. When the pump starts getting starved of inlet volume, then this is when pumpability issues begin.

This is also true for the W rating pumpability viscosity defined in SAE J300. At some point, the cold oil's viscosity is just too thick for the forces on the oil in the sump to move it effectively to the pump. If the oil is very slow to get to, or can't get to the pump at all through the pick-up, then there will pumpability problems and lack of lubrication to the engine. Cavitation will most likely be going on too, which hurts pumpability and could also cause damage to the pump itself.

Now imagine a bucket full of a fluid like water or oil.

If atmospheric pressure pushed "down" on the fluid then it would weigh more at high pressure and less at low pressure. But atmospheric pressure doesn't affect its weight. It weighs the same as ambient pressure rises and falls.
If you had a super accurate scale that could measure down to a very low weight increment, the bucket would actually weight less if all the air above the oil was removed (ie, a vacuum) from the system, because air does have weight. The air pressure on everything on Earth is due to the total head pressure of the thickness of the whole atmosphere (60 miles thick) surrounding Earth. If you could slice out all the air above the bucket 60 miles high and weigh it, it would weight quite a bit. But since the bucket is completely surrounded by air, the pressures cancel out. Maybe this concept is what you're locking onto which might be confusing the issue when talking about PD pump operation (?). Imagine what the bucket would weigh if you could somehow remove all the air just located below the bucket but still have 14.7 PSI pushing down on it. Do the calculation - ie, 60 miles of air head pressure, which would be 14.7 PSI pushing down with no reactive force on the bottom of the bucket ... it's surprising.

Now poke a hole in the side of the bucket near the bottom. The rate at which the fluid drains through the hole does not depend on ambient atmospheric pressure. Ambient pressure pushes down on the fluid in the bucket, and also pushes in from outside the hole, equally. No net effect or impact on the rate of drain. It drains at the same rate regardless of ambient air pressure.
If your example is an open bucket, then yeah the ATM pressure won't matter because the force on the water is ATM pressure + head (gravity), and the hole outlet is going to ATM. So only the head pressure of the water depth is the driving force to move the water out the hole in the side. But a PD pump does not work that way.

Now say the bucket is an engine oil sump and connect the hole to a PD oil pump. If lower air pressure didn't slow down the drain rate through the hole, how can it affect the intake of the pump?
A PD oil pump is not like an open bucket with a hole in the side. In your bucket example, there is no additional delta-p between the water on one side of the hole and the ATM beyond just the head pressure of the water.

The inlet of a PD does not operate at the same pressure level as the outlet ... far from that. The inlet of the PD operates basically at vacuum (suction) dependent on it's design and RPM (regardless of what the pressure on the pump outlet is) which makes the delta-p between the oil in the sump and the pump inlet greater than what's going on in your bucket example. The greater the delta-p, the greater the driving force to move the oil from the sump to the pump inlet. If you could magically remove all the air pressure on the oil in the sump and only left the head pressure from gravity, the driving force would go way down, and if the oil was cold/thick enough it would start impacting the oil flow through the pick-up tube and into the inlet side of the pump. At some point pumpability would be effected. If you could then magically increase the pressure on the oil that's pushing it into the pump inlet (all other factors held constant), then that same thick oil would move better to the pump inlet. It's the pumpability of the oil for that specific pump, which can be effected by a many factors - and the ATM pressure due to elevation is one of those factors.
 
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ZeeOSix

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The problem is, having university physics and a degree in math, I am over-thinking this. Otherwise I'd be satisfied with the intuitive notion that ambient air pressure "pushes down" on the fluid. Of course that seems obvious. But if it were the true explanation, then why doesn't a bucket with a hole near the bottom drain slower at high altitude?

So while I accept the conclusion that ambient air pressure does indeed affect the pump, and this does explain the original question of why Ford recommends a lower viscosity oil based on altitude/pressure not just temperature, I'll continue to noodle on this to see if I can convince myself of the true reason WHY this is the case. I'm sure it's something simple that I'm overlooking.
I think you will see it if you do a bit more research on how PD pumps work - you can't use the bucket analogy because it's not even close to how a PD works on an engine. There's tons of info about their operational characteristics at your fingertips.
 

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To add ... the only time a PD would act like the bucket analogy (in post #85) is if there was zero resistance on the PD pump outlet and it was putting the flow to the atmosphere. In that case - which never happens in an engine - then the ATM pressure would not effect how the pump worked if the ATM pressure on the inlet side was the same ATM pressure at the outlet side. But in typical use, PD pumps are putting their output through some kind of resistance, and the pump outlet is operating way above ATM pressure. But the inlet is still operating at a suction/vacuum, and the forces on the fluid feeding the pump does have an effect on how that fluid flows into the inlet side of the pump. Maybe that will help with a missing piece of the puzzle.
 
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... Imagine if you could pressurize the sump to 65 PSI of air pressure (50 PSI above ATM). Pressurizing the sump would help push that oil more effectively through the pickup tube to the inlet of the pump which would still be at the same vacuum level. ...
Agreed there. With the sump pressurized above ambient, we have created a pressure differential which exerts a net force on the fluid.

... The inlet of a PD does not operate at the same pressure level as the outlet ... far from that. The inlet of the PD operates basically at vacuum (suction) dependent on it's design and RPM which makes the delta-p between the oil in the sump greater - regardless of what the pressure on the pump outlet is. The greater the delta-p, the greater the driving force to move the oil from the sump to the pump inlet. ...
Yes, I acknowledged this earlier. The pump inlet is below ambient, its outlet is above ambient, and the power delivered to the pump determines the rotational speed of its rotor and the pressure differential from intake to output.

... If your example is an open bucket, then yeah the ATM pressure won't matter because the force on the water is ATM pressure + head (gravity), and the hole outlet is going to ATM. So only the head pressure of the water is the driving force to move the water out the hole in the side. But a PD pump does not work that way. ...
Now we approach the nut of this. As soon as you put a hole in the bucket, you allow ambient pressure to act through the hole on the fluid inside which equalizes the ambient pressure pushing down on the fluid. Pressure equalizes and gravity is the only remaining net force causing fluid to flow out the hole.

... A PD oil pump is not like an open bucket with a hole in the side. In your bucket example, there is no additional delta-p between the water on one side of the hole and the ATM beyond just the head pressure of the water.
Right. With a sump and oil pump, there is no hole. Ambient pressure has no path to push against the oil entering the pump intake. It can only push against the surface of the fluid in the sump. Ambient pressure is compressing the oil against the inner walls of the sump even though its volume doesn't change since it's a (relatively) incompressible fluid. But it is under pressure. Suppose for simplicity the pump is not moving. The pressure at that inlet rises and falls with atmospheric pressure, just like it does in the bucket without the hole.

Put differently: the sump is getting squeezed from all sides, inside and out, from ambient atmospheric pressure. But the oil is "caught in the middle" squeezed against the inside walls of the sump.

To add ... the only time a PD would act like the bucket analogy (in post #85) is if there was zero resistance on the PD pump outlet and it was putting the flow to the atmosphere. In that case - which never happens in an engine - then the ATM pressure would not effect how the pump worked if the ATM pressure on the inlet side was the same ATM pressure at the outlet side.
Exactly, and that is the point I overlooked. The oil entering the pump intake isn't going to ambient atmosphere. It's entering a chamber of the oil pump gear/rotor. The reason I overlooked this is thinking about atmospheric pressure on the output side of the oil system. This adds to the pressure, which adds to the force against the rotor at the oil pump output. This force should be transmitted through the rotor to the intake side which would equalize the portion of pressure that came from ambient, leaving only the pressure differential from the pump. However, [and this is the simple key point I overlooked] the pump motor opposes or bears this additional pressure from the output side, so it cannot act on the oil entering the intake.

Put differently: in the bucket with a hole, if you stick a pipe in that hole so the fluid drains through the pipe, and you put a freely spinning rotor in that pipe, it doesn't change the fact that it drains at the same rate independent of ambient pressure. Ambient pressure from outside acts through the rotor and pushes on the fluid, equalizing the pressure on the fluid surface. But connect a motor to that rotor and everything changes. You can apply power to speed up or slow down the flow. Powering the rotor prevents outside ambient pressure from acting through the rotor on the fluid inside. The powered rotor counters that force so it cannot act on the oil at the intake.

... But in typical use, PD pumps are putting their output through some kind of resistance, and the pump outlet is operating way above ATM pressure. But the inlet is still operating at a suction/vacuum, and the forces on the fluid feeding the pump does have an effect on how that fluid flows into the inlet side of the pump. Maybe that will help with a missing piece of the puzzle.
Indeed.
 
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ZeeOSix

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Double post ... see post below which is complete.
 
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ZeeOSix

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Agreed there. With the sump pressurized above ambient, we have created a pressure differential which exerts a net force on the fluid.
And likewise, with a sump at higher elevation where the air pressure is less, there is a less pressure differential between the sump and the pump inlet that's at a vacuum level.

Yes, I acknowledged this earlier. The pump inlet is below ambient, its outlet is above ambient, and the power delivered to the pump determines the rotational speed of its rotor and the pressure differential from intake to output.
What also determines the pump inlet vs outlet differential is the resistance that is connected to the pump outlet - like an engine oiling system. A good PD pump will literally destroy things if its output pressure is not regulated with a pressure relief valve. As said before, on a well designed healthy pump, the inlet and outlet are basically separated and operating independently. The output can go from ATM (0 gauge pressure) to 100s of PSI gauge pressure, and the inlet will still basically be operating at a vacuum/suction. Only when a pump is worn badly then there can be lots of "pump slip" past the rotor(s) between the outlet and inlet sides of the pump which can effect its performance.

Now we approach the nut of this. As soon as you put a hole in the bucket, you allow ambient pressure to act through the hole on the fluid inside which equalizes the ambient pressure pushing down on the fluid. Pressure equalizes and gravity is the only remaining net force causing fluid to flow out the hole.
In the bucket example, the head pressure also decreases as the level of fluid decreases. If you put a PD pump on that hole in the side of the bucket, the pump would flow more fluid than a hole could, depending on the size of the hole, because the pump would be causing a suction/vacuum on the hole which would increase the relative delta-p driving force (head + ATM pressure) on the fluid in the bucket. That would make the fluid flow better if it was thick enough to where more driving force filled the inlet of the pump better/fully (ie, better pumpability). When a PD inlet chamber(s)) volume can not be 100% filled, and the incoming flow volume can not keep up by filling it 100% as the pump speed increases, then the pumpability of the fluid (like cold oil) is less than 100%.

If an engine was cold started in very cold weather, the oil may make it to the pump and pump well, but if someone revved the engine up pretty high right after a cold start-up, the pump could certainly be starved if the oil is too thick (not 100% pumpable). In other words, the oil may pump good at 1000 RPM, but no so good at 3000 RPM in those cold start-up conditions.

Right. With a sump and oil pump, there is no hole. Ambient pressure has no path to push against the oil entering the pump intake. It can only push against the surface of the fluid in the sump. Ambient pressure is compressing the oil against the inner walls of the sump even though its volume doesn't change since it's a (relatively) incompressible fluid. But it is under pressure. Suppose for simplicity the pump is not moving. The pressure at that inlet rises and falls with atmospheric pressure, just like it does in the bucket without the hole.

Put differently: the sump is getting squeezed from all sides, inside and out, from ambient atmospheric pressure. But the oil is "caught in the middle" squeezed against the inside walls of the sump.
I like to look at it like this: When the PD pump is running, the pump pick-up tube becomes a suction area at the bottom of the oil in the sump. The head pressure of the oil (due to gravity) plus the ATM air pressure on top of the oil surface provides the driving force to move the oil through the pick-up tube and into the pump inlet ... all fluids will move from higher to lower pressure. So the oil will flow into the pick-up tube and into the inlet side of the pump rotor(s). If less volume flows into the inlet and leaves some air/voids in the pump inlet chamber(s), then the pumpability of the fluid is less than 100%. Once trapped in the rotors it will be forced out the pump outlet, even if it takes 100s of PSI to do so. PD pumps are monsters, and therefore need pressure regulation.
 
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@MRC01 and @ZeeOSix, I am ploughing through this discussion with interest, and plan to reread it in its entirety, but am just jumping in here to commend you both on your civility. The discussion been about you both truly trying to understand Ford's apparently strange oil-viscosity, rather than resorting to ad hominem attacks. Well done.
 
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@MRC01 and @ZeeOSix, I am ploughing through this discussion with interest, and plan to reread it in its entirety, but am just jumping in here to commend you both on your civility. The discussion been about you both truly trying to understand Ford's apparently strange oil-viscosity, rather than resorting to ad hominem attacks. Well done.
@Alien, you too!

I too am baffled by Ford's requirement, but don't have much to contribute.

It certainly appears that Ford thinks there's something beyond temperature influencing the oil pumpability.

Hope you sort it out.
 
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@Alien, you too!

I too am baffled by Ford's requirement, but don't have much to contribute.
It certainly appears that Ford thinks there's something beyond temperature influencing the oil pumpability.
...
Yes, and Ford is right. As the oil pump operates, it creates a low pressure cavity near its intake in the sump. At low atmospheric pressure, the oil in the pan fills this cavity more slowly. This is exacerbated by cold oil as it is higher viscosity and more sluggish. So, above a certain altitude they recommend using a thinner oil to prevent the oil pump intake from drawing air on cold starts.

That is the answer to the OP's question. Yeah we're on page 5 but we finally got there!
 
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I'll add that according to Ford's recommendation from the OP, 0w30 is OK for all temperatures so you can simply use 0w30 all the time and not worry about high altitude.
 
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This whole high elevation pumpability factor never crossed my mind until we had a "deep dive" into the subject.
Same here. What is interesting about it: most people assume ambient atmospheric pressure "pushes down" on things. For example most people assume that a bucket with a hole in the side drains faster under high atmospheric pressure. They forget to account for the fact that ambient pressure doesn't just push down, it is everywhere and pushes outward in all directions, which includes pushing back through the hole into the bucket.

In this case of the oil sump and pump, that over-simplified incorrect assumption leads to the right answer for the wrong reasons. And if you know enough to consider atmospheric pressure on the opposite side of the pump, you can get the wrong answer for the right reasons. What I find interesting about this is it's one of those cases where getting the right answer for the right reasons requires a bit more subtlety of thought.
 
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They should say elevation. Altitude is when planes fly lol
Same difference. Ford always gives thorough explanations on there oil requirements/recommendations I've learned.. in 15yrs+ of buying there trucks both light duty & heavy duty for business purposes .
 
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