I've been running Castrol VMX80 in my gearbox because that's our workshop supplier but the change to Redline MT90 has improved the shifting feel a great deal. It slots into gear much easier now and the bearing noises have decreased too. Good shit. Expensive though.
EDIT: Had a trackday running this fluid and it made a world of difference. Rolling back down the pit lane (after very hot sessions): the bearing whirr from the box running Castrol VMX80 was almost non-existent. Highly recommended.