How to Determine the Correct Oil Grade for your Car

OVERKILL

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Thanks for the info!

Just to make sure I understand this correctly, based on J300 table, 20W Min cSt can be between 5.6 and 9.299 (< 9.3). Is that true?

if true, 9.3 also happens to be borderline Max hot rating cSt for 20 and 30 grades (i.e. it could be a very thick 20 or a thin 30) ... is that why you are saying 20W hot rating might be 20 or 30?

Q2:
was the 9.3 under the Min and Max cSt columns of Widman table by design or just a coincidence? What's the story behind it if any?

Looks like Zee beat me to it :)
 

AEHaas

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Very true. Some mechanical problems like oil pressure loss can be caused by wear in engine bearings for example, from using an oil that was too thin. I prefer having a nice safety margin built in, instead of having "adequate protection."
A safety margin is not one ended. Too thick an oil, especially at higher RPM, may result in the worst type of wear within moving parts, cavitation. This is when chunks of metal are pulled off of the surfaces. If the oil is a little thin and the moving surfaces get close then the additive package comes into play to prevent or minimize any wear. Nothing can prevent cavitation if the fluid cannot keep up with the movement of parts.

AEHaas
 
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A safety margin is not one ended. Too thick an oil, especially at higher RPM, may result in the worst type of wear within moving parts, cavitation. This is when chunks of metal are pulled off of the surfaces. If the oil is a little thin and the moving surfaces get close then the additive package comes into play to prevent or minimize any wear. Nothing can prevent cavitation if the fluid cannot keep up with the movement of parts.

AEHaas
This true and there are always trade offs. My homework lead me to a 30 grade for my two Jeep applications. IMO it offers the best of both worlds, especially since they weren't specifically built for a 20 grade oil only, and the spec was more for CAFE credits than anything else.
 
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Very true. Some mechanical problems like oil pressure loss can be caused by wear in engine bearings for example, from using an oil that was too thin. I prefer having a nice safety margin built in, instead of having "adequate protection."

As far as wear rates are concerned over a long period of time, yes. Sudden catastrophic bearings failures though are often the result of an issue with the crankshaft such as excessive torsional vibration or crank walk/warp. In such cases, you could have Nitro 70 in the bearings and it won't prevent a thing.
 
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As far as wear rates are concerned over a long period of time, yes. Sudden catastrophic bearings failures though are often the result of an issue with the crankshaft such as excessive torsional vibration or crank walk/warp. In such cases, you could have Nitro 70 in the bearings and it won't prevent a thing.
That's true regarding a catastrophic failure. Wear can also result in a catastrophic failure as well.
 
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The point I'm getting at is we (very) often overthink this matter. Yeah, you may see slightly less bearing wear with a 5w-30 or 5w-40 in an engine spec'd for 5w-20. Now the engine may last 700k miles instead of 600k miles. It doesn't really matter when the vehicle will likely go to the scrapyard for some other reason with <300k miles on it.
 
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A safety margin is not one ended. Too thick an oil, especially at higher RPM, may result in the worst type of wear within moving parts, cavitation. This is when chunks of metal are pulled off of the surfaces. If the oil is a little thin and the moving surfaces get close then the additive package comes into play to prevent or minimize any wear. Nothing can prevent cavitation if the fluid cannot keep up with the movement of parts.

AEHaas
I could see that in Siberia, and I agree it would be a problem. But at my ambient temps it will never be a problem. Plus the fact that main and rod bearing clearences have not gotten smaller than when 5w30 was common use.
 
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A safety margin is not one ended. Too thick an oil, especially at higher RPM, may result in the worst type of wear within moving parts, cavitation. This is when chunks of metal are pulled off of the surfaces. If the oil is a little thin and the moving surfaces get close then the additive package comes into play to prevent or minimize any wear. Nothing can prevent cavitation if the fluid cannot keep up with the movement of parts.

AEHaas
Of all the papers and studies I've read on lubrication and tribology, cavitation between moving parts is never discussed ... why is that? Most cavitation concerns are at the PD oil pump. Anyone have some technical papers addressing cavitation between moving parts, like cam lobe and a valve follower or between piston/rings and a cylinder wall?

And besides, I highly doubt there is going to be any cavitation potential difference between 20 to 50 weight oils when they are at operating temperature. If there is cavitation due to oil being too thick, then every oil viscosity is going to have cavitation as it goes from a cold start to operating temperature, especially in very cold weather conditions.
 

AEHaas

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We know that the rate of engine wear follows the curve of oil viscosity. At start up (75F for test engines) the wear is highest and it gradually decreases as the oil temperature gradually increases over the 20-30 minutes it takes the oil and engine to warm up to the normal operating temperature. Then wear is at a steady state and at its minimum. If the thicker oil at the lower engine temperature has no ill effects then why is wear higher? If thicker oil lubricates better than thinner oil why is there more wear.
The answer cannot be that the engine parts are not fitting together well at start up. The oil lubricates or it doesn't.

One of the main reasons I like thinner oils is that they are not too thick during the start up interval. In Florida at mid summer the temperature may be 90F. This is COLD for your engine. The definition of a cold engine is not sub zero temperatures. Actually it (cold) is more like 75F for engine test purposes. A 0-20 oil may start out at 100cS mid summer when I leave the house. And yes, most trips are less than 30 minutes so the oil never reaches the appropriate operating viscosity of around 10 cS. The least engine wear does not occur when the oil is at 100cS as the "thick" people would lead you to believe. The least wear occurs when the oil is up to the normal operating temperature.

AEHaas
 

4WD

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Of all the papers and studies I've read on lubrication and tribology, cavitation between moving parts is never discussed ... why is that? Most cavitation concerns are at the PD oil pump. Anyone have some technical papers addressing cavitation between moving parts, like cam lobe and a valve follower or between piston/rings and a cylinder wall?

And besides, I highly doubt there is going to be any cavitation potential difference between 20 to 50 weight oils when they are at operating temperature. If there is cavitation due to oil being too thick, then every oil viscosity is going to have cavitation as it goes from a cold start to operating temperature, especially in very cold weather conditions.
Cavitation is generally written about wrt diesel cooling systems … pretty rare in lubricant fields … but

 
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If the thicker oil at the lower engine temperature has no ill effects then why is wear higher? If thicker oil lubricates better than thinner oil why is there more wear.
The answer cannot be that the engine parts are not fitting together well at start up. The oil lubricates or it doesn't.



AEHaas
Cold start up wear? The fuel mixture is richer. This affects the cylinder walls. Especially true with DI engines. My mazda had >5% fuel after 8 mile trips which is my wife's typical commute.
0w20 and 5% fuel is not what I want.

And does this cold start up data show where the wear occurs?

Is it main bearings? Ring and cylinder walls? Camshaft? Oh or ohv? Timing chain?
Startup wear sounds generic.
There are differences in thermal expansion, especially pistons. I.E. forged vs cast . The will be a bit more blowby when cold till the pistons and rings expand. Hence the ring gap rule of .003-.004 per inch of bore.
There are known engines that have piston slap due to this, like some gm LS engines.

Piston rings now sit higher on the piston than years ago, this is for emissions and effiency. Therefore the top rings see hotter temps and a suffient gap is required. This gap allows some blowby when cold.


Last which is outside my knowledge is additive activation.
Do the antiwear additives in any grade oil work cold? Perhaps boron. But the others?
 
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AEHaas

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"Search,read,and study some of Shannow’s papers."

Search, read, and study some of Dr. Eric Schneider's papers who has performed most of the oil research for GM, as one example. There are numerous SAE papers on start up wear.

AEHaas
 
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"Search,read,and study some of Shannow’s papers."

Search, read, and study some of Dr. Eric Schneider's papers who has performed most of the oil research for GM, as one example. There are numerous SAE papers on start up wear.

AEHaas
How about some links to said papers, since you know all about them.
 
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How about some links to said papers, since you know all about them.
 

AEHaas

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Getting back to 10PSI/1000 RPM:
I reviewed an article provide by E.J.M. Slaats: Loaded Plain Bearings, April 2007.
Here are some observations: They state that the oil hydrodynamic (HD) pressure is the sum of the oil feed pressure and the intra bearing HD pressure. Since the system feed pressure is so low compared to the HD pressure, the bearing Force is the same even with a system feed pressure of zero. This goes along with what I have said in the past. The oil pressure is not the determining factor for lubrication in a bearing. The effect is basically zero. The pressure only exists to move oil into the bearing as it is lost from the side of the bearing. My comment - As long as the system pressure is 10 PSI for every 1,000 RPM then oil flow should be sufficient to keep the bearing wet in a properly working SI engine. I cannot reproduce the equation here but see item 2.4.1. In order to keep the force, F, in the bearing a constant value while increasing the clearance, C, (more leakage of oil) you need to logarithmically increase the oil flow, Q.

My point is that the oil pressure has nothing to do with lubrication or wear in a bearing. Flow is important, pressure is not. Viscosity and film thickness are still important because of the spacing needed for dirt, soot and to keep the asperities separated. The paper did note that the combined roughness of the journal and bearing made oil film thickness a factor. But after the break in period, there was much less film thickness needed. They stated the roughness value for the bearing surface actually becomes zero so it is only the remaining journal roughness that comes into play after this time period.
 
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Getting back to 10PSI/1000 RPM:
My comment - As long as the system pressure is 10 PSI for every 1,000 RPM then oil flow should be sufficient to keep the bearing wet in a properly working SI engine.
It all depends on the specific design of the engine's oiling system, because the oil pressure delta per 1000 PRM increase is dependent on all kinds of things - ie, overall oiling system flow resistance and design, oil pump volumetric output and efficiency, oil viscosity & temperature, etc.

Have you done oil pressure vs temperature vs engine RPM measurements on your Ferrari with different oil viscosities used? If you used 5W-20 and saw less oil pressure delta vs RPM it's because of the decrease in viscosity ... but theoretically, the PD oil pump would still be supplying the same volumetric oil flow at the same RPM if the pump isn't in pressure relief. In that case, the engine is still getting the same oil flow volume, but the lubrication system is running on lower viscosity and therefore less HTHS and less MOFT between parts.
 
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