A new synthetic lube on the market, one that is little-known at the moment, is "Kellube M-12." Developed by pistolsmith Neil Keller and a chemist buddy of his, this stuff is one of the best rust preventatives I've yet run across.
Kellube is red in color and is thick and viscous. It applies like no other "gun oil" I've ever used; a little truly goes a long way.
To quote from the bottle:
"...provides a 100% increase in protection over conventional petroleum gun lubricants."
"...displaces moisture and coats metal with a protective film...."
We started using this stuff exclusively in our shop about a year ago and now won't use anything else. If you can't find it locally (which you probably can't), it can be obtained from Neil Keller at Kustom Ballistics (219-724-2879). We (SM&A) also carry a stock of it, in four-ounce bottles, for resale. It's also now available, I believe, through Brownells.
BTW, rust is pervasive -- once it sets in it's very difficult to stop without removing it entirely and refinishing. Prevention goes a lot farther than cure in this case.