Oil viscosity for our buicks

This seems like a silly question, but what is the oil viscosity everyone is using? I ask because I've had different sources give me different answers. From engine builders to shade tree mechanics, they all had slightly different responses. I "think" I've been using 10w-40 from what an old engine builder told me. Thoughts?

I would think the consensus is to use the thinnest oil that provides adequate pressure when hot.  You want 15-25 @ idle and 10psi for every 1000 rpms above idle as a rule of thumb.

With that said I've been using Brad Penn 10w30 -- it's important to note that newer oils do not contain as much zddp as they use to so be sure to either use a special oil ( like the Brad Penn ) made for old cars OR add zddp additive ( can use blue bottle STP oil treatment - its the cheapest of the additives ).

I use rotella t 5w40 synthetic. Swear by it in my turbo Saab and also the Buick.

Thanks for the replies, seems like I have some choices. It's good to know there are a few people running different oils (and additives) with positive results.

l also use rotella but 15-40 for me. never had any problems after all these years.a lot of vintage cars ride with this particular oil because of a few reasons....do your research.