about the only reason to use straight weight oil is momentum from the past, even Caterpiller recommends 15/40 now. You will have to ask Grig about Detroits, I know 15 years ago they were recommending non-detergent 40 weight, but maybe they have caught up with the rest of the world by now. I have been using 5w30 for about 5 years now just because thats what all my new cars are using it seems to be working fine. the engine was rebuilt in 1979, form your own opinion.
