Redline MTL. Period. We've used their products in our race FC's for many years without problems and there's no reason to change. Shifting is smoother, transmission is quieter and aggressive driving, IE: 9500 rpm shifts are no trouble...

Regards,
-Markus

I personally use Royal Purple Max Gear, great stuff. Shifting is smoother overall, especially in first and second, with the Max Gear as opposed to Redline MTL and MT90.

Lucas Heavy Duty 80/90 Gear Oil
Figured I'd try a non-synthetic that works wonders with everything else.