I coat the exterior of my guns with Renaissance Wax and I've never had any kind of rust problem. Keep in mind that any oil used is permeable and probably won't keep moisture from your firearms.
It is typical that some of the used firearms I acquire have surface rust on them. I remove this using a product such as Flitz before coating them with Renaissance Wax and the rust has not returned.
Jim

The 2nd Amendment IS an unalienable right.