Lubriplate (White automotive type grease) on the gears and any light weight oil on the gear bushings / driveshaft bearings. I've been using the same high-speed bearing oil bottle for 3 years. Stuff lasts forever and is perfect for tanks. The stuff you want probably comes in a small bottle with a long thin needle tip.

Otherwise try to keep flying grease out of the rest of your tank, and keep wires out of your gearboxes!