Originally Posted By: CATERHAM
Of course having oil gauges allows one to do away with having an excessively high viscosity reserve with no risk once you become comfortable relying upon them.
But the manufacturers need to specify an oil grade will a large viscosity reserve to deal with all the potential unknowns such as fuel dilution, oil shear and unexpectedly high oil temp's that could happen when one is driving blind so to speak.
The last factor, unexpectedly high or abnormally high oil temp's manufacturer's do have more of a handle on now than they once did through the use of electronic safety management controls which has enabled lighter oil grades to be specified.
So given the option, that's why it's best
to run the lightest oil a manufacturer specifies
and if that is heavier than a 0W-20, and you have oil gauges, you will know if can run something lighter with the way you operate your car.
While I agree that your oil pressure/High shear viscometry correlation is sound, again, the OEMs are using their minimum oil pressure recommendations on their RECOMMENDED lubricants as an indicator of when equipment needs mechanical intervention.
It's not the "minimum" oil pressure to target viscosity selection.
The pressure and temperature gauges cannot "see forward" into what's happening in the big end bearings for example. The big end temperatures are MUCH MUCH higher in temperature than the bulk oil temperatures, and only intermittently oiled depending on feed hole locations and main bearing grooving.
I know you've discounted on a number of occasions that the temperature rises across the bearing because of oil shear, rather than the misconception that it's "carrying away" combustion generated heat, but it's there.
Piston cooling squirters (where fitted) are pressure driven, so targetting lower oil pressure is intentionally reducing squirter flow....and even Honda have stated that one of the challenges with low viscosity induced oil pressure is maintaining operation of cam phasing hydraulics.