I went to a friend's house once and fetched a couple of old shitguns from her attic. They were serviceable, but had developed a this coat of rust, enough to require re-bluing.

I believe the heat cycling (heating + cooling + condensation) in an attic acts on guns the same way auto exhaust acts on a car's exhaust system.

There's enough oxygen floating around in various forms that the _lack_ of an oil layer can allow rust to form. There are other ways to treat it, as your uncle alluded to, but they are probably very sensitive to following the appropriate maintenance schedule.