I seem to be having a lot of problems finding any non-synthetic 80W-90 for my rear end. What kind of non-synthetics is everyone using and where are you getting it?

I am afraid to use Mobil 1 because I have heard it is too slipery for the clutches it the posi. I just have the standard posi in my SS.

Would the Valvoline durablend be to slippery for the posi?

Has anyone ever used Castrol or Penzoil gear oil?

Thanks for any suggestions.