Hey OP... I posted above from the perspective of having owned a 1971 B30 (one ton window van long wheelbase not maxi) for 21 years. Replaced 318 engine with 360, changed transmission and timing set a few times...
Oil... Owner's manual offered several Oil Choices. I think the main ones
were 10W-30 and single weights according to a temperature scale. Towing our boat, I found less oil loss with 30 HD oil. Even tried 20-50 Castrol (to inventory less oil types for our home "fleet") and the 30 worked best. I know you probably don't want to run straight 30 on the Michigan end of a winter trip...