Originally Posted By: Hokiefyd
Originally Posted By: Brit33
Quote:
Yes, but manufacturers do not recommend an oil pressure -- they only recommend an oil viscosity
totally untrue.
Alfa Romeo specify oil pressure for idle and max revs.
Your statement and Gokhan's are both true -- and at the same time. You are correct: nearly all manufacturers specify a minimum and sometimes a maximum oil pressure at one or multiple points in the engine speed range. And Gokhan is correct: the owner's manual will almost never say, "ensure you use an oil that results in a hot oil pressure of 10 psi at 1,000 rpm".
I said "almost never" because I suppose there may be an outlier somewhere, but in the vast majority of situations, the manufacturer recommends a GRADE of oil. They do also specify the oil PRESSURE in service-related information so technicians can diagnose the lubrication system if there is a problem, but this is rarely, if ever, a recommendation made to an owner.
There is actually one correction to what Gokhan said: manufacturers don't recommend oil viscosities, they recommend oil grades. A viscosity is "10.5 cSt", and often given at a certain temperature, such as "at 100 deg C". A grade is "10W-30".
Thanks, that's exactly what I was trying to say!
Certainly, I meant SAE viscosity grade by the viscosity recommendation.
SAE viscosity grade specifies a range for the 0 C and 40 C kinematic (low-shear) viscosities and imposes a lower limit on the HTHS (high-temperature [150 C], high-shear) viscosity, where the oil flows through microscopic gaps between sliding parts (high shear) at high temperature. The low-shear (kinematic) viscosity, where the oil flows through macroscopic openings, is the pumping viscosity, which determines the oil flow and pressure. However, these days some manufacturers make an implicit yet more restrictive recommendation on the HTHS viscosity as well, by specifying a specific ACEA Ax/By category or the API Cx category, which impose additional lower and upper limits on the HTHS viscosity.
The two SAE viscosity numbers xW-y dictate oil flow and pressure in cold and hot engine, respectively, whereas the HTHS viscosity dictates the minimum oil-film thickness (MOFT), which in turn dictates bulk of the fuel economy (inversely proportional to HTHS viscosity) and protection against wear caused by high load (high vehicle speed, towing, or climbing), low RPM, and/or dirty oil (directly proportional to HTHS viscosity).