Understanding why different viscosities are recommended?

jrh

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wisconsin
New user here. I read the motor oil university on this website but I need some help with something and I'd appreciate some insight.

Something I've always wondered was why do manufacturers recommend different high temp (say 5w30 or 5w40) weights for their vehicles. If I understand correctly, the viscosity grade should be based on bearing clearance for a particular engine. If the operating temperature is constant, this should logically just be a single grade at operating temp, with winter grade varying for different temperature ranges, correct?

For instance, here is a chart from a 2006 f250 6.0l powerstroke diesel I have:
[Linked Image]


How come 30 grade and 40 grade are recommended for different temp ranges when the operating temp should be the same regardless of the climate?

Also, because motor oil is never thinner than operating temp at cold start, is there a reason why you *wouldn't* want to go with a 0 winter grade (like 0w40) even if the manufacturer specifies a 15w40, no matter what climate? Is there something about the spread between the numbers that make a 0w40 oil less stable than a 15w40? or will it break down faster and not guarantee a 40 weight?

Sorry if my terms are all wrong, new to this but I couldn't find answers to the questions above.
 
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The majority of the wear occurs during start-up, therefore operating temperature is not a factor and I see why they favor 0W30 at low temps. But why 0W-30 is not okay for 90 degrees same as 10W30, well you got me.
 
The operating temperature of an engine does change with climate though, at least in terms of oil temperature. On cars that I have had with oil temperature gauges you can see a significant difference in the average operating temperature on the coldest day of winter vs the hottest summer day.
 
Originally Posted by Patman
The operating temperature of an engine does change with climate though, at least in terms of oil temperature. On cars that I have had with oil temperature gauges you can see a significant difference in the average operating temperature on the coldest day of winter vs the hottest summer day.

It shouldn't make that much of a difference as long as the cooling system is working.

Of course many of the recommendations for higher temperatures and/or towing assume that the cooling system may not be able to keep up with the oil temps.
 
A lot of different things. One is that some weights are assumed to be less durable, even though there can be different 5W-30s that can last or not in that application. I guess with a chart like that it's impossible to simply account for every possible oil type.

I've heard one common claim that for most cars a SAE 30 would be ideal where the manufacturer spec's a 5W-30 or 10W-30 as long as the block is kept warm with a block heater. A lot of these requirements have cold starts in mins where something like a SAE 30 can be really difficult to start and might not pump well in freezing temperatures. But I guess the other thing about SAE 30 is that they allow things in it with the latest API specs where a multi weight must have less to avoid damaging catalytic converters.
 
Originally Posted by ka9mnx
That is an EXTREMELY confusing chart!


Nah, not if you break it down into pieces:


Ford intends for 10w or 15w oils to be used, until the temperature drops below a certain level; 15w-40 for trailer towing is self explanatory, stronger film strength and thicker film protects under maximum stress.

Below freezing, looks like they want a 5w.

Below 0°F/-18°C is the only place they want a 0w.

Aside from that it appears either a 30 or 40 is acceptable. I'm not sure that 0w-40 existed in 2008, or it probably would have been included beside the 0w-30.
 
Originally Posted by Patman
The operating temperature of an engine does change with climate though, at least in terms of oil temperature. On cars that I have had with oil temperature gauges you can see a significant difference in the average operating temperature on the coldest day of winter vs the hottest summer day.

I don't know about diesel pick up trucks but I worked on class 8's. The big trucks ran cold in winter despite thermostats and radiator shutters. They just never warmed up to the thermostat temps and the 2 cycle Detroit's ran a 140F!
 
This engine is a little different than most in the way the fuel system operates, which uses engine oil. We have always had good luck running 5W40 synthetic in them.
 
Originally Posted by JLTD
Aside from that it appears either a 30 or 40 is acceptable. I'm not sure that 0w-40 existed in 2008, or it probably would have been included beside the 0w-30.

Back then there were a lot more gasoline/diesel rated motor oils. I do remember maybe by the time the API SL or SM fewer gasoline motor oils had an API CC/CD rating. Before then many "conventional" gasoline motor oils had already lost their diesel ratings.

However, Mobil 1 0W-40 has been around since at least 1996, although it might have been more a European car thing. I first used it around 2006 because Mobil was recommending it as suitable given that 5W-40 was the factory fill in my WRX. Mobil mentions it here:

Quote
https://mobiloil.com/en/article/why...ifference/brand-heritage-mobil-1-history
Porsche turns to Mobil 1â„¢ 0W-40 synthetic motor oil as a factory and recommended service fill for its 1996 models.
 
Ambient temperature and radiant surface temperatures have a notable impact on oils operating temperatures. This is a reality of physics and the cooling system can only somewhat blunt this. Cooler temperatures allow more oil temperature to be radiated through the passing air over the oil pan, at hotter temperatures less heat is transfers to air and the cooling syste, helps some, but the sump temp will go up as the oil is only momenatarily in contact with the water jacked of the cylinder and at a very small volume. With a coolant/ oil exchanger there is more contact time and volume, but it is still only momentary and the physics of the ambient air temperature will prevail.
This chart shows what is preferred. Above freezing 15w40, below freezing to -10f 10w30.
 
I'll bet it has a lot to do with HPOP pressures.

I think I read somewhere the injectors won't fire unless the computer sees the correct HPOP pressures.

That event will occur sooner with thinner oil in the winter...

I think they are trying to avoid "excessive cranking times"??
 
Originally Posted by BigShug681
In terms of your 6.0 PSD just run a 10w30, the injectors will shear down 40w to 30w in quick order





Very Interesting.
 
Originally Posted by jrh
How come 30 grade and 40 grade are recommended for different temp ranges when the operating temp should be the same regardless of the climate?

They might be basing that on using the most basic version (inexpensive multiweight) of a 5W-30 or 10W-30 along with the assumption that the lower weight base oil will break down under the stress. I've seen a few owners manuals that only recommended 10W-30 unless there was really cold temps. My dad's 1996 Buick Regal with the 3800 Series II said 10W-30 was the ideal oil, and that 5W-30 was OK if was under maybe 50ºF? It also recommended a "synthetic 5W-30" for really cold temps. I think in a year or two they were already recommending 0W-30 for cold temperatures.

The original Acura NSX and the Honda S2000 strictly had a recommendation for 10W-30. They didn't really specify any performance requirement other than the current API gasoline specification.
 
Originally Posted by PimTac
Originally Posted by BigShug681
In terms of your 6.0 PSD just run a 10w30, the injectors will shear down 40w to 30w in quick order





Very Interesting.

It is, the oil in this system sees pressures of around 3000psi at WOT firing the injectors. I've had 40w turned into 30w within 3k miles
 
Originally Posted by y_p_w
given that 5W-40 was the factory fill in my WRX.


Umm, y_p_w, exactly what WRX do you have? 40 weights have NEVER been factory fill for Subaru, not even in the STI. It's ALWAYS been factory 5W30 with the EJ engines, and the FA engine calls for 30 weights as well for the DITs, 20 weights for the FA20 in the BRZ. You "can", and many people have, use 40 or even heavier weights in an EJ, but they don't roll off the line in Lafayette or Gunma that way.

OP, PSDs are always a conundrum. You need an oil that will flow quickly when cold so the injectors don't get wonky, but also thick enough (or low in VII) so that the HPOP and injectors don't shear it down and cause accelerated wear. The "easy" choice would be to use the Motorcraft 10W30 rated for PSDs, but that may not be enough if you tow heavy in high ambient temps. I believe Rotella T6 5W40 will help meet both of ends of your operating spectrum, but don't plan on extended OCIs if you do tow heavy.

Get two OCIs on the same oil under your truck's belt, then send in a UOA to see how the oil is holding up ("wear" metals are essentially useless; UOAs in industrial settings know that UOA is not a wear tool, it is a "condition of the oil" tool). It will tell you if the oil will tolerate the load, temps, and length of miles/time that you are subjecting it to.

And finally, OP, remember that bearing clearances are just a number to be maintained by the oil- at hot temps, you need something "thicker" to maintain the minimum oil film thickness (MOFT), where in cold weather, the "heavy" stuff will be much too viscous to pump efficiently and you risk starving the rotating assembly, valvetrain, and injectors of the stuff that keeps metal parts separated. This is where you use the "lighter" oils, so that it is more fluid at the colder temperatures. Remember thermodynamics- if the ambient temp is very high, and the engine is a certain temperature, there will be a given maximum amount of heat the radiator can get rid of, along with radiation and convection around the block, heads, and oil pump. In a very cold ambient temperature, the amount of heat that can (and will) be removed by the various channels of heat extraction WILL be greatly increased... just like Patman's observations prove.

Welcome!
 
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