I try to tear them down after they get used at a game. Some people don't lube until there's a problem.
I use 25wt 100% pure silicone oil on almost all parts. GBB's and AEG's. You can even use heavier weight like 75wt. The silly oil that comes with the AI propane adapter is lighter and I tend to use that more in mags. On occasion, I'll use dabs of white lithium grease in mechbox's.