for the m20 i recommend Delco GL 4 gear oil (special order at any GM dealer)......anything else eats up the brass in your trans.....the 12 bolt 80w90 Valvoline (works great in my ride)
If it's a posi, the GM specific posi oil (sorry I can't get you tell part number at the moment ) and no additive needed if you use that. For the Muncie 80/90 works great except in January!
__________________
1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
If it's a posi, the GM specific posi oil (sorry I can't get you tell part number at the moment ) and no additive needed if you use that. For the Muncie 80/90 works great except in January!
Have not found the all in one GM fluid only the additive Mark posted
I'm back at work after a couple of weeks off and found the number. 89021672 is specifically for posi before the synthetic days. However, it's discontinued. About 25 dealers in the country still have some stock. I have a note in my book that I wrote, Penzoil 550042083 is the same oil apparently.
__________________
1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars