I think the longer this post is up on the board, the longer you will get opinions on what to run in the T-5 tranny you have. Here is my two cents.
I'm running a 85-86 S-10 T-5. When I wanted to find out what to run in it, I came to this board and at that ime I never could get a definitive answer - like this thread. I decided to go to eBay and by an Owner's Manual for an 85 S-10 Blazer and see what the manual said. It said ATF Dexron III which is now Dexron IV.
Case closed.
