Well I am getting close to my first oil change on my 401 (well actually seconed, first was right after I broke it in) But I am thinking about switching to mobil 1 synthetic. I was wondering what weight would be best in mobil 1. Right now I have castroil 10w-40 with good results. So I was looking for that weight in mobil 1 but couldnt find it. The closest I could find was 5w-40, made for trucks and suv's, or something like that. I guess it being a little thinner when cold would help with start ups? Mobil has all kinds of funny weights. Is there any that would be the best for AMC engines?