I was told that quaker state 4x4 ATF works great. its a synthetic blend. Jeff Bordelon told me this. He is the man who sold me my tranny. I have seen him post a few times over at Clubcobra.com under the name Trannyman Jeff.
The viscosity on the bottle says its consistent with that of gear oil, but it looks and pours just like automatic trans fluid. I've read of alot of people using it with good results.
I know with mine, I put this new T5 Z in and it was notchy, even after a few thousand KM. Powershifting was out of the question, I almost wanted to yank it out and return it... A few people recommended MTL. Bought some and put it in, and after about 150 KM with it, it immediately started to smooth out in how it was to shift. I can crank on it all day and powershift now when I am at the track.