Bryan,
I called Richmond before I put anything in my Super T-10 and asked what I should use. They have their own oils but they also recommended, which I am using BFL Manual Trans/Rear End
Synthetic Blend by Lakewood Industries, part Number 32950. Lakewood are the people who make the bellhousings. It is a 75W-90W blend, it looks like automatic trans fluid (red). The one thing Richmond told me is do not use any
oil that has an additive like for a limited slip diff. in it.