For what is worth my engine builder says that the heavy weight oils are harder on your engine. He has done some extensive testing in this area and found that most damaged caused to an engine is in the first few seconds of start up. Lighter weight oil circulates faster and helps avoid the damagcaused by a slow flowing oil. Bottom line is that the lighter oils get to the top faster and help protect earlier. This is a theory from Joe Sherman Racing Engines. Makes sense to me.