Read on another forum that the detergents in the rotella multi vis is excellent for the tappets.
The viscosity depends on the condition of the engine.
I am not the expert, but if the thing maintains good oil pressure≈lower multi vis numbers/colder climates and SAE 30-40-50 for warm climates
Do some more internet research and decide which way you want to go.
With aircraft we always used phillips X/C until we got a new engine, then went with shell multi vis. It was air cooled, but the technology is 40's era for both.
JP