Here's a website to check out and a quote:
"One last piece of important information. ALL World Class T-5 use ATF oil and NOT heavy gear oil or Redline MTL! All NON-World Class T-5 use 70 wt gear oil. We recommend high quality Synthetic oil as it lasts a long time and over a larger operating temperature."
"A WC T5 takes about 2.75Qts of ATF, where as NWC T5 use 2.75 Qts of 70 weight gear oil. Tremec 3550/TKO takes 3Qts of GM syncromesh only. We recommend that you change the oil in the transmission once every two years for street driving and once a year for severe driving."
http://www.moderndriveline.com/Technical_Bits/t5_history.htm I think alot of people have heard over and over again 't-5's take atf' so they repeat it. I don't think its true though. The NWC T-5 CAN use ATF but they were designed for 70w gear oil/syncromesh.
HERE's a great step by step article with pictures on how to rebuild your t-5. Note at the beginning of the article where it says:
"Compared to NWC T5's the WC's benefit from needle bearings under the gears and tapered roller countershaft bearings for reduced friction. This change also allows the later to run considerable less viscous ATF fluid rather than heavy gear oil."
http://www.fordmuscle.com/archives/2005/12/T5Rebuild/index.php If you still have doubts, why not drop by the local FLAPS, open a Chilton's or Haynes for a '83 s-10 and see what it says for the 5-speed lube. I havn't done it, but I'll bet it says 70w gear oil.
I use synthetic 70w syncromesh, I think its a Valvoline product
