What kind of trans fluid was being used before you added the Amsoil? I've always assumed that trans fluid is trans fluid, but maybe the different fluids weren't compatible and it caused a problem?

Or, maybe adding the Amsoil had the same effect we hear people talking about when changing from regular oil to synthetic oil in engines, where the synthetics start breaking up old deposits and it gets into the new oil and new filter or causes gasket leaks?

If the trans is already not shifting and the option is not driving it at all, maybe it would be worth trying something like a Seafoam Trans flush (If Seafoam makes such a product), then draining the fluids and changing the filter again? Cheaper than a new transmission anyway, but I don't have much experience with transmissions, so anyone else reading this feel free to nix the idea if you think it might just make matters worse.