The Severe Gear and Longlife Amsoil Gear lubes are GL-5/MT-1 rated, which means they have excellent compatability with brass/bronze syncros. However, they are intended primarily for differential use. Amsoil also just introduced a new GL-4/GL-5 rated, marine gear lube in a 75w-90 grade, so I'm not surprised by the new MT fluids. I'd expect one to be a GL-4 only, SAE 75w-90 and the other to be a direct GM "syncromesh" replacement. This would be formulated more along the lines of an SAE 5w-30 engine oil in terms of high/low temp viscometrics ....
TS