on my t5 swap i just did i used atf. thats because i assumed thats wat it takes as the v6 t5 cars and t56 thats ive had ised atf. but i read rom 83-87 they used gear oil. i kno the trannys older cuz its got a speedo cable on it. but i cant find anywhere on autozone or advanced that says the older style takes gear oil. i even checked alldata at my work.

 

ATF was done for mileage and works fine.

If you're going to push a lot of power through it at high speed, worry about synthetic ATF at the expense of synchronizer function.

If you're just going to run a healthy 305 or 305 @90mph, don't worry about it.

 

They all use ATF....I know my old Haynes manual said to use 80/90 gear oil back when I had my old BW T-5, and I did. Mistake. I think Haynes made a mistake because the trans grinded and shifted like crap. I used ATF after that, all all was fine.

 

GM, in different years, specified ATF, gear lube, and motor oil; ALL WITHOUT ANY CHANGE WHATSOEVER TO THE PARTS THEMSELVES. My 83 service manual for example calls for gear oil.

Which tells me, basically ANY non-volatile petrochemical with lubricating properties is probably OK.

Best stuff I ever used in any of my DOZENS of those old T-5s, was half-and-half STP and ATF. Unbelievably slick shifting. Didn't make em last one mile longer, or less long, that I could tell, for that matter. Leaked out the stretched case around the front countergear bearing at about the same rate as gear oil. Pure ATF leaked out the fastest... would basically fall out of most of em onto the floor overnight.