Ditto on the previous reply. Dexron II or III. The following is pasted from a search for "T5 transmission" and then "tranny lubricant" on cj-8.com. Long thread that I can't seem to capture a URL for....
My T5 is in pieces on the garage floor with 3 bottles of Dexron III nearby.
Oh Boy! The nightmares are returning...
Back in '86, I removed my crippled T4 (filled with 80/90w gear oil) and replaced it with a T5 from an '85 CJ7 (filled with ATF). Both my '81 FSM and an '85 FSM said 80/90w gear oil, but then again my '84 Z/28 w/T5 used ATF. I called a few shops, 1 said ATF, 2 said 80/90w, and the last one (who professed to having rebuilt more than 30 T5's) said it calls for ATF, but if I wanted it to shift smoothly AND last a while, to put a high quality 5/30w "detergent" motor oil in it. That was the last straw!
I called the gear division of Borg-Warner and told one of the engineers the same story written in the paragraph above. He told me that in late 1981, B/W started recommending ATF over the 80/90w gear oil to help the tranny shift smoother (due to customers complaining of "notchy" shifting), especially when it was cold. They found that the "heat-dissipation properties" of ATF were almost as good as the "thicker" 80/90w gear oil, and would be plenty for the aluminum-case'd T4-T5 since Aluminum retains less heat than Cast Iron. ATF also absorbed less moisture and was thin enough to provide an overall improvement in shifting (ie..smoother). Plus, since it coated metal surfaces more quickly during cold startup, it resulted in a minimal (though measurable) decrease in the type of wear normally caused during the first few minutes of operation.
Ok, this guy sounded pretty sure of himself so I've been running ATF in my T5 (yes, that same one I installed back in '86) ever since. No problems yet. (add the sound of wood being knocked on here)
HTH
Nick
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'81 CJ8. Full Hardtop, 258(0.030") I-6, T5, RV Cam, Borla Header. F&R D44's w/4.09's & ARB's. 4" Lift, 33x12.50x15 BFG MT's, Mud
2001 Camaro SS M6. Onyx Black/Ebony Leather
Dynojet #'s - 334rwhp/356rwtq