In response to the title, water based lube for mi....I mean uh...
Anyway, all joking aside, silicone oil is good for gas guns to keep the seals dry and putting on the OUTSIDE of the hop rubber if it's a tight fit. Also good for the slides on gas pistols and putting new canisters in CO2 pistols etc.
Bar that, you'll want Lithium grease or so for the metal on metal parts