call TTC and ask. sometime in 1999 or 2000 the synchro ring material was changed so there's less of a chance of any fluid used attacking it.
according to TTC they ship the t56 factory-filled with mobil-1 atf, and say they have the best results with it, even though we all complain
I drained the t56 on my 2002 SS around 8000 miles this past spring. shifting was getting lousy and 2nd gear was becoming a problem. The fluid looked almost brown coming out, maybe bad factory fill ? anyway I tried gm synchromesh for about 2000 miles and it works great. By the way, I know of another LS1 guy running synchromesh in his 1998 f-body t56 for more miles than me with only good things to say. So I seriously doubt the synchromesh fluid is going to hurt the t56 (a synchromesh transmission) even thought the sticker and manual say to use dexron3 atf.
I drained out the synchromesh last week (for warranty reasons because it's not red colored and car is going into dealer for leaking rear pinion seal) and filled it up with cheap $1/qt proline tranny fluid from pepboys. It shifts just as good as it did with synchromesh. I really should have drained the factory fill and used the cheap atf first, oh well. I also found out part of my shifting problem was my hurst billet; I put the oem one on there and it's as smooth as can be.
However it is warm out now, over the winter the shifting was horrendous when it was cold. with either shifter. I will be trying out redline hi-temp atf (dexron3 rated) soon, have it sitting in the garage, but I will run the proline for a couple thousand miles to get my use out of it since it works well now. The redline hi-temp atf has the same viscosity of synchromesh and redline mtl, around 10.0 cSt. Regular atf, mobil-1 or amsoil atf are below 8 cSt. I'm guessing this might be one possible reason for some notchy shifting problems, I'll know better once I use the hi-temp atf. Will have to wait till winter too to see how the higher viscosity fluid does when cold.
I wouldn't recommend mixing MTL with MT-90. I would go with either redline mtl or regular off the shelf dexron3 atf or the lucas before GM synchromesh simply because synchromesh is $10/qt only from the dealer and a dealer will have every right to void the tranny warranty if they want to. You can also find pennzoil/havoline synchromesh fluid in stores for around $6/qt I think.
If the lucas stuff is dexron3 and you're okay with the cost, then I say at least try it. If you don't like it then it's easy enough to drain and refill with cheap dexron3 atf. And I wouldn't worry too much about heat in a manual gearbox, especially the t56- it's huge with an aluminum case. Heat is only a significant factor in automatics that are high horsepower or drive a lot of weight.
like colin quin says, that's my story and i'm stickin' to it
hope this helps.
