No, not saying that. There are quite a few people that run it, and like it. I can not say that I have done any type of testing to say that the Syncromesh is bad for teh T56. All I can say is, teh T56 is designed to work with thinner based ATF. Thicker oil like teh 'Mesh, can provide an added "cushion" so to speak, in higher mileage trannies. Can give you an example, but of a T5.
A LONG time ago, had a T5 in a Mustang start to make alot of noise, alot of whine. So, I drained teh ATF that it called for, and added 10W30 motor oil. made it ALOT quieter, so, more is better, right? So, drained it, and added gear oil (like 80W90 or something), and all noise gone, AND it shifted great.. Now, i drove that tranny with gear oil in it for @ 10K miles, and never had a lick of problems with it after I added teh gear oil. Would I have eventually? Not sure. Car got totaled
So, I guess what I am saying is, the 'Mesh IS thicker than ATF, but not much, and would THINK that it would be fine to put in for long periods of time.