Start here.

http://6066gmcguy.com/gmcv6a.html

I have a 305 and use Shell Rotella straight 30 weight, designed for diesels I think but was turned onto it by another V-6'er

1960 GMC 305A V-6 Gallery Page
Remember, the government cannot give --anything to anyone --that they have not first taken away from someone else.
BUY AMERICAN http://www.stillmadeinusa.com/index.html