Gear fluid

[Dbakk1104]

So I am installing 3.42 ring and pinion gears along with a posi unit in my factory gm 7.5 inch 10 bolt rear end (1978 cutlass) and I’m curious what kind of fluid I should use. I have seen some people say 75w140 and 75w90. Recommendations?

 

[1320chevy]

So I am installing 3.42 ring and pinion gears along with a posi unit in my factory gm 7.5 inch 10 bolt rear end (1978 cutlass) and I’m curious what kind of fluid I should use. I have seen some people say 75w140 and 75w90. Recommendations?

you can use either one , the heavier of the two generally for severe service and towing. Make sure you add posi additive though to prevent the clutches from sticking.