Just curious about synthetics. Not a "my oils better than your oil" arguement. I want to know whick ones use a base oil 4 and which ones use base oil 3. I know AMSOIL does, thats a given, but what about Rotella T6, Mobil 1, Royal Purple.... etc? I don't seem to read much about Royal Purple on here either. Thats one I don't hear people mention much.