There have been quite a few posts on this engine, including my own. (L67). Turns out the 3800 is very easy on oil, and just about every oil has turned out great results. Chevron/Havoline works great for a dino, and the M1 and GC results look great too.
I use 10w30 in the summer and 5w30 only for the coldest months(below 0).
-T