If your running BG syncroshift, MTL, or GM syncromesh straight, transmission won't last too long as these are too thin. They should be mixed with thicker fluid.

I stay away from MTL and MT 90. Prefer regular 75w90 GL4 gear oil instead.

I do my own blends now, and have been sticking with the same formula for a while

Search around and everyone has a different opinion.

The best thing that will make your transmission shift smooth, is a good transmission and properly adjusted clutch.