I've been messing around with StickyG grease sets (moly lube gear grease and silicone cylinder grease)...Airsoftparts.ca carries it. Nice stuff, the gear grease is very slippery. Excess tends to fly off the gears. The cylinder silicone grease is pretty thick...and doesn't "liquefy" at temp (the cylinder can get pretty hot under long full auto bursts). One set is enough to do several full rebuilds.
Other than that I've just used white lithium grease for everything...works fine.