Lubing the gears is done to decrease friction as the cogs rotate, and effectively rub on each other. This allows the gears to remain cool under heavy usage (due to low friction caused by the slimy, thick viscosity of good lube), which in turn maintains the gears' structural integrity. Without the lube, there would be more friction, and in turn heat buildup, + more noise thrown into the mix.
I use "super lube synthetic grease" for gears simply because of its good viscosity, ability to be used on rubber and plastic, and seemingly excellent adhesion to the gear cogs even in high rof setups. The stuff just lasts a LONG time and doesn't get thrown around too much.
HOW AND WHERE TO USE GEAR GREASE: You need to use gear grease in several areas. first, I like to put a little bit on my finger and smear it carefully between each gear cog. More is not necessarily better, but it is certainly worse than too little grease. If you get to much on, it'll just fling off on the first shot and make your GB a little more 'messy' lol.
Next, I'll take a little dab of grease and smear it on all the axles prior to inserting them into the bushings. From there, you can put an EXTREMELY small "film" of grease on the piston rails, but you must be careful since this can cause fps loss if your grease is too thick. It is not necessary to put any grease on your sector teeth which contact the piston, but a little won't hurt if it is plastic safe stuff...
I think I covered most of the stuff
