If you put in ATF into a Tremec box and got better shifting performance, then you most likely lucked out, and/or the original fluid was in fairly bad shape.

ATF is designed for a very different kind of transmission, which uses a torque converter.

ATF, MTF, Engine Oil - all different.

Use an actual MTF, like GM Synchromesh or Acura MTF, and if you really feel like spending $$$, you can use stuff like Royal Purple, Amsoil, etc.