Using the correct grease inside your Airsoft AEG's gearbox is an often overlooked part of properly tuning your rifle. So, in a rare demonstration of my capability to actually be brief, I'll provide you with some very important information on proper lubrication for your mechbox in just a few words.
I. Lubricating the inside of your AEG's cylinder and the piston head o-ring seal:
Use Silicone Oil.
It's the same stuff you use in your GBB Pistols to preserve the many seals associated with that type of gun.
Silicone oil is better for situations where there are rubber seals involved, since it doesn't deteriorate the rubber like the types of grease that are recommended for use on gears.
II. Lubricating your AEG's gear set:
There are a number of effective greases, but I've only tried a few. I started with a basic White Lithium grease that you can find at your local hardware or auto parts store. It worked well enough, but then I started reading more about PTFE/Teflon and decided to try a White Lithium grease with PTFE. The grease I got is made by a company called Fuchs. It just made too much sense to go with the PTFE, given its highly effective lubrication characteristics, especially in my high-torque setups where there is a substantial load being placed on the gears.
Do not use the same grease on the cylinder/piston head seal that you use on your gears. As I've said many times before, they are two different systems that require two different types of grease.
There. That wasn't so bad, was it?