It's Synchromax (not Synchromesh) High Perf. MT Fluid. I swapped it into both center boxes last Friday. I was not enthused by the grit in the drain pan when I emptied the transmission, but it seems to be a really nice oil after driving it a few hundred miles.

Even with age and wear, shifting is smoother through the 5 and in and out of 4wd. It pulls 80 pretty easily again. The only disconcerting thing is that there isn't a weight listing on the bottle and is recommended for vehicles that use ATF as well as weighted and unweighted motor oils. It pours like a 75/90. Their site probably gives an equivalent, but smoother shifting and better power delivery seem to sell it for me.