ALLLL T-5s use dexron mercon (older model GM auto fluid) atf fluid...do not run 90 weight gear oil in them.......not type F ford auto fluid...its no trick,,its what they came with originally...
lost of guys also run GMs synchromesh trans fluid which can be had over any GM parts counter...its essentially a synthetic fluid designed for Gm version T-5s and T56s...
Borg Warner and Tremec recommend using old style Dexron II/Mercon.
Some people use synthetic or newer type ATF. For some people it seems to work for some not. In many cases it works fine for a while then later (like six months or so daily driven) they find the gears begining to grind a bit when shifting. This is because many synthetics are a bit TOO slippery. The WC T5 has carbon fiber blocking rings. These function like miniature brake shoes. The function of blocking rings is to slow the next gear to close to the speed of the shaft so the synchronizer and gear you are engaging don't grind. If you know how to "double clutch" you can get by without the blocking rings because you would be performing the same function. (All a basic and simplified explanation).
The upshot is that old Dexron is generally the safeest bet, particularly if you want to speed shift. If you tend to drive your car rarely and shift rather slowly and gently, you can run synthetic or what have you and likely will never experience gear clash.
Note that some gear clash going into reverse in most T5's is unfortunately pretty normal and what fluid you choose to run will likely have no effect on it.
I personally know of a healthy 185K mile T5 that has been running B&M "Trick Shift" fluid since about 1986. I can't swear that the B&M fluid was benificial or not, but it sure didn't seem to cause any harm. I've tried Trick Shift in other type manual transmissions for like 20K miles or so and couldn't really tell any difference or benefit.
I put Dexron III/Mercon in my T5's and Mercon V in my AOD's, but that's just me.
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