What's that got to do with your statement in post #55 of: "... these positive displacement pumps are all but a thing of the past in passenger cars, as engineers/designers look to improve parasitic losses in engines."Agreed the positive displacement (gerotor or lobe or gear) is at the pump proper, and the pump must lube itself, but that isnt it's purpose. It is to lube plain bearings and component proximal and distal in the engine proper through discharge, but will be limited in ability to move lubricant volume by the viscosity of the fluid when bypass is initiated. Volume flow though a restricted orifice at constant pressure will be lessend with increasing viscosity.
The oil pumps in modern cars still move the same volume of oil regardless of viscosity (as Shannow mentioned) just like the old time PD pumps do if not in pressure relief.