Weight does depend on the engine and the clearances it is built with. The 996 porsche cup motors run 0/30 mobil1 as the factory recommended race oil. I normally stick with 15/50 for my stuff and am really not concerned about the 1 or 2 HP gained with a lighter oil. You could save more HP with proper baffling in the pan than you will save by getting too light on the oil viscosity.

Joe