How to Determine the Correct Oil Grade for your Car

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I have been away from the site for years. It seems now I have some time on my hands as I am close to retirement.

Anyway, it is simple: The rule is that you need 10 PSI for every 1,000 RPM of engine speed. I generally like to keep it as 8 - 12 PSI.

In a thread somebody was asking what grade to use. He stated that he mostly drove “normally” but on occasion was on the track. He got all sorts of advise but the correct response. He needs two oils, one for each application. He needs to use whatever oil gives him the appropriate PSI for his different applications. It’s that simple.

FYI, my Ferrari 812 Superfast normally runs at 35-45 PSI at idle (650 RPM) at normal operating temperature while driving around town, my only application. The oil temp has never been above 170F so far. The oil is a 40 grade but surely I can use a 20 grade safely. The maximum oil pressure is limited to 80 PSI at 9,500 RPM because of the pop off pressure valve. So Ferrari thinks that 80 PSI is enough for 9,500 RPM. They do caution that the engine must be fully warmed up before running up the RPM - Why? They do not want the oil to be too thick with higher RPM.

AEHaas
 
I have always heard the 10psi per thousand oil pressure rule. I get that you need a minimum oil pressure move the lube.

But does oil pressure separate the parts? I don't think it does. Its moft that keeps the parts apart. The crank does not float on oil pressure. According to your theory, any weight oil will work as long as you have oil pressure. Will a 2cst oil work at 9500rpm with 50 psi?
 
The anti-wear additives such as zinc respond to heat and provide better engine protection when the oil is warmed up vs being cold.
Titanium is a good anti-wear additive when the oil is cold and hasn't warmed up yet.
 
I have always heard the 10psi per thousand oil pressure rule. I get that you need a minimum oil pressure move the lube.

But does oil pressure separate the parts? I don't think it does. Its moft that keeps the parts apart. The crank does not float on oil pressure. According to your theory, any weight oil will work as long as you have oil pressure. Will a 2cst oil work at 9500rpm with 50 psi?
Yes, the only function of oil pressure is to move the oil to the parts and faster flow will cool the parts better as well. Cooler parts will have a thicker oil and cooler parts wear less. Minimum Oil Film Thickness (MOFT) increases with RPM (all else being constant) so it is fortuitous that higher RPM is protected by increasing MOFT.

AEHaas
 
10psi per 1,000RPM is just a MINIMUM rough rule of thumb that was originally used by guys building SBC's, it has little to no applicability to other applications, particularly engines with OEM clearances, nor should it be used as a general guideline in that regard for viscosity selection. The last stock SBC I saw that had 10psi at 1,000RPM had no rod bearings left in it (standard clearance motor, tired).

If you were building an SBC and HOT at 1,000RPM you weren't seeing 10psi or 20psi at 2,000RPM, 30psi at 3,000RPM under load then you had probably built it too loose. You could then choose to step up in viscosity until you did. It could also be used to tune the other way, but this was typically used for loosely built race engines, not ones that used OE clearances. An engine built to OE clearances on spec viscosity will always have MUCH higher oil pressure than the old SBC "rule of thumb".

This is why Honda went with wider bearings when they dropped down to spec'ing 0w-16. There's more to safely being able to run a thinner oil than just observed oil pressure, these are engineered products.
 
Owners manuals aren't written by engineers. They're written by marketing and lawyers, for the most part. Otherwise how to explain the fact that two exactly the same engines, with exactly the same internals, that are operated in similar climates, have different oil recommendations simply due to the country where the vehicle was sold? Example: Sahara isn't much different in climate than Death Valley, but oil recommendations will be different in owner manuals between US vs African Republics, even if the vehicles came off the same assembly line on the same day and hour, with the same exact engines.
Owner manuals are blanket statements and compromises. Do your homework for the BEST option for your engine.
 
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