The which oil thing is an interesting question. In my earlier days I was a bearing designer. Internal combustion engines use a combo of hydrodynamic (bearings) and boundary layer (cylinders, valve guides, lifters) lubrication. Across the board I would say that as an engine wears more viscosity is better. If you are in a hot environment and like to punch it a 20W50 is usually appropriate. However the synthetics behave better than the old oils. A 5W30 synthetic may in fact perfrom as well as Castrol GTX 20W50 in terms of wear and still reduce friction compared to the 20W50.

I will break my 502 in on 5W30 dino grease and then switch to 5W30 synthetic at 1000 miles. GM swears the 5W30 is the way to go. That seems a little light for my taste, unless it's synthetic .................

Kitz