I have seen both of them used and have been told by numerous people that one is better than the other. But no reasons were ever given.

 

ATF is necessary for WC T-5's only. NWC T-5's use gear oil. It has something to do with certain materials used in the syncros or something that is the same as an automatic tranny and doesn't react well to gear oil. But it's only for the WC T-5's. There are peolpe on this board that use gear oil or a mix of ATF and gear oil in WC T-5's without a problem, but I think I will stick with the manufacturer's recommendations. Anyway, it should say right on the tranny which to use.

 

run down to a dealer either GM, fNord, or Dodge. Ask them for thier synthetic manual transmission gear oil. Use that instead of atf or gear oil. from your chevy dealer its called Syncromesh, Hundia's use an SPF-5 ATF which you could also use if you wanted a thinner oil and your transmission is a newer T5. Agh just get the Syncromesh

 

Ask your dealer first, or get a repair manual or check there. Don't have much experience with T5s but one I helped rebuild had Dextron III recommended it had the paper type blocking ring materials though. Make sure 100% though that you put the right stuff in there.

 

Wait a minute...I thought the original poster HAS a T-5...if it calls for Dexron III, use a synthetic ATF...if it calls for gear oil, use the Synchromesh or BG Synchroshift...I have experience with the BG Synchroshift and it made a world of difference in shifting and trans feel, especially when the trans was cold

 

I've read that T5 synchros are made for ATF, that there are drain holes in them sized specifically for that viscosity, and if you run 90 wt. it will mess up their timing.
Long ago, when we gutted V8 Monzas to build V8 Vegas, some of the Monza 5 speeds had ATF in them and some had 90 wt.--all identifiers and internal parts were the same, so we thought we'd put 90 wt. in all of them to make them more durable--wrong! The ones that came with ATF would bind up if you put 90 wt. in them.
The Chevy dealer there in Sacramento told us to run whatever we found in the tranny when we found it, that they were the same mechanically but you had to keep running whatever lube it was broken in with. We never had a problem running ATF in one of those little 5 speeds behind a 327 or 350, long as you didn't jump on it in first too hard.