[MadMax]

I agree with tte in that there isn't a good definite answer to how much grease to use.

In the few gearboxes I've designed (for other purposes) we've packed them with grease because there was no expectation that they'd be serviced by users and lifecycle testing showed that having grease was good. Unfortunately the situation isn't so simple with AEGs. We have some odd issues atypical in most gearbox designs.

We've got electrical switch contacts which can become fouled with burnt grease because we sometimes get arcing in the carbon block. We've also got a piston/cylinder assembly which can blow grease down the hopup and barrel assembly which can result in shots going all over the place.

It may be more useful to specify lubricant level in various regions:

Inside piston/cylinder assembly
-inside cylinder: very light wipe of light grease (light lithium or lightweight silicone oil) inside cylinder to reduce dynamic friction in piston head oring
-on piston rack: light application of light grease (lithium)
-light wipe of lithium in piston rails

On gear teeth:
-medium application of lithium grease before assembly, not more than 1mm thick beads on alternating tooth flanks. Rotate the gear to scrape off a little grease so it's picked up in 1mm thick beads between teeth. The grease will pick up on the approaching flanks but not on the retreating flank when you do this, but it'll smear out when the gears mesh.

Inside bushings and gear axles:
-heavy application of lithium grease, pack bushing/bearings apply a medium film (~0.5mm thick) to axles before reassembly.

Misc sliding surfaces like tappet plate:
-light wipe of lithium

White lithium grease is pretty good for our gearbox application. It's not the best for heavy tooth contact, but I like it because it's not so tenacious that you can't clear it from a barrel. I've worked with some very sticky greases that would be hell to swab out of a barrel without solvents.

Don't bother with silicone oil (save for the cylinder) as silicone oil has no surface tension. Even very heavy silicone greases will continually spread and escape your gearbox. Silicone lubricants are good for corrosion protection because they seep into cracks and corners, but crap for metal-metal contact because it has no film strength.

Occasionally when I feel like getting dirty, I modify my lithium grease by pooting a little colloidal graphite into the grease. You can find colloidal graphite in Canadian Tire. It's a dry graphite powder that comes in a little plastic tube, usually used as a dry lubricant for locks. I like the stuff because the graphite particles smear out and provide some low velocity lubrication which is good for tooth engagements. Greases and oils can only prevent metal-metal contact by floating parts apart from each other when lubricant flow causes hydrodynamic effects which push the rigid parts away from each other. This does not occur in gear tooth engagements because we start and stop rotations so frequently (starting with metal-metal contact) and we have such high contact pressures. I like a little voodoo graphite to provide a graphite layer to reduce the metal scraping on metal in the gear teeth. This is outright speculation though as I haven't run a lifecycle test in an AEG mechbox to see if graphite helps. It's damn messy stuff though.