Beeman used to sell a dry form of moly-sulfide powder as a cylinder lubricant for their airguns; they may still or not. At one point on the network54/FunSupply boards there was a discussion of if it was worth using or not. The arguments against it were that it was would absorb water over long periods of time, resulting in rust. It is a good lubricant in absolute terms, but it doesn't protect against rust damage (and the claim was that) it may even help cause that. SO it is only really good for machinery that (due to ambient temperature conditions) is under no risk from moisture accumulation.
I haven't been able to turn up info on the matter elsewhere, but I no longer use the stuff on my airgun. The lube in question was dry-powder and the instructions stated that if you applied it dry, it would bond to metal as-is. It was never stated to be a rust-preventative, so some people may have been misusing it. On a firearm a bit of rust is an annoyance, but on an airgun, a little bit of rust can mean an expensive rebuild so it was more of an issue in that use.
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