I agree 100% that engine oil must be topped up and clean. I was taught that in High School shop and stands almost any logical test that can be applied.
But I have never received an explanation why my three beaters that should have stopped running at some point in time did not! If the theory that proper, quality, clean and full oil makes an engine run a long long time, then should improper, alegedly poor quality, certainly dirty, but always full oil result in a grenaded engine? The theory is wrong; I proved it wrong.
A lot of people here claim one must use synthetic and one must change it often. Often there are arguments as to which brand is the only brand to use. My actual experience has disproved all those. And let me tell you, that 3.0 litre Chrysler motor was at redline while pulling the trailers. For many, many miles. I did have to put the heater on for a while because I saw the temp gage climbing.
And I would say that there are not major differences in crankshaft loadings and oil shear loadings in most engines regardless of RPM/howrsepower/torque ratings. Mercedes, Nissan, Plymouth, Chevy, Mercruiser, Crusader, Caterpillar, Cummins, Mann, MTU, even high speed deiesels like Yanmar. The same oil is used in a 3500 HP 16 cylinder Mann diesel as in a 90 hp 4/5 cylinder diesel engine made by Benz or VW or any other.