Originally Posted by
onehundred80
I think that using the oil pressure gauge to decide what oil grade to use is not of any great use.
You have stated before that the pressure goes up as the revs increase, this is to be expected as the pump cannot push the same volume of oil through the small clearances and holes at the same rate as it can when the revs are lower. This is because the clearances and holes have not changed while the volume of oil that is trying to be pushed them has increased in proportion to the increase in revs, ignoring small inefficiencies, therefore the back pressure increases and the oil gauge gives a higher reading. Having a high reading could mean that insufficient or no oil is being pumped through the small clearances only through the holes because the oil is too thick.
What you want is oil able to get through the small clearances no matter the temperature of the oil and engine. Also the oil must be able to adhere to the surfaces under load and not get squeezed out.
In general, I do not disagree, Dave. In fact, the specification for our engines is for "3 pounds or more" oil pressure at engine idle at 205F. (My engine never gets above 195F but I still saw pressure of 5-6 pounds with 0W40.) And I would never put 20W50 or even 10W40 in this engine (altho MB does approve of a few 10W40s in 229.3.)
From my perspective, 5 weight oil is pretty damn thin. For the vast majority of the time the engine is running, lubricating oil is meant to keep bearings in suspension at 200F (or higher), I simply do not agree with 0 weight oil in most cases. When I started driving, 10W30 was the most commonly sold and used product out there, my new truck in 1993 specified 10W20, which I ran up to 10,000 miles. After that, I went to 15W40 and after 100,000 miles, I went to 20W50. That engine is still on the road, at over 460,000 miles (actually, it was taken off the road due to rust in the last several years - but it is still used to haul wood around a farm). The block, valves, crank, rods and rings are original. And yet, I was told I'd ruin the engine if I went past 10W30.
I do admit that engine startup wear is a concern - but I am not one who leaves my vehicles outside, and I tend to let them warm up before driving when the temperature is below even 50F.
A basic understanding of the SAE viscosity numbers is a good start.
I'd like to see you expand on that. I've read a lot about SAE multi viscosity ratings, and a lot of what is published is contradictory.