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If I understand the science correctly, the higher 40w oil should have a thicker film or boundary layer of lubrication at operating temp. If the issue is surface finish (picture mountains/valleys) ...and that finish making contact with the bearings/crank journal...a thicker film might be able to...

Not disagreeing with you philosophically, but GM and their engineers have said that a large part of the oil viscosity and type are based on bearing clearance specs and that the Duramax oil pump combined with the clearances that the oil must flow through would not work with a thicker oil...

Why would the Duramax be so different from the euro turbo diesels? The total volume of testing and data across Mercedes, BMW, VW, and others, is massive…more than GM has published by far. GM even certified several 5w30 oils as Dexos D.

I’m far from an expert, but it looks like some of the Euro oil tests and standards are as high as or higher than Dexos D. Why would something like Pennzoil Euro 0w20 be incorrect vice Mobil 1 0w20? Similarly, there are 5w30 Dexos D oils…and Euro 5w30s with very similar specs. They seem...