Be wary of any gear oil with a GL-5 rating on the label. They can make the gearbox feel notchy, and worse, may contain compounds that can harm the "yellow" metals in your transmission's bushings and synchronizers.

In my limited experience I've found only 2 aftermarket oils that work well in the X manual transmission. They are:
1) a 50/50 mix of Redline MT-L and MT90 and 2) Amsoil MTG.

I'm sure there are others, too.