Rem oil = mineral oil.
I would change what you're using. You can't go wrong with CLP, it
Cleans,
Lubricates
and Protects.
Mineral oil is OK for cleaning only because it's less viscous, but it's not good at preventing corrosion.
Rust can be like an infection... you may have to use a good penetrant like Penetro 90 to like disinfect it - root all the rust out:
http://www.schaefferoil.com/penetro-oil.html
Penetro 90 has this stuff Scheafer calls "Rust Ender" and from what I've heard on the lubrication forums, it is pretty good at treating and preventing rust.
Is the rust on the outside? If you've gotten rid of the rust and you have a good coating of CLP on it and the CLP soaks in, or Penetro, you can take a cotton cloth and wipe basically dry, and then put a coat carnuba wax on the your shotgun's outside. I haven't tried any of the advanced polymer waxes but they supposedly have advanced long chain-polymers that have improved hydrophobic qualities. On a car that means being in the rain or going through car washes multiple times and still beading the water. Probably a coat of carnuba car wax after you have the proper lube soaked into the metal will work.
I'm guessing that anything that is safe for the finish of a car is safe for the finish of a gun.
You can also get "Rust Ender" separately from Scheaffer.
Anyway, it's not the Mossberg, it's the Rem Oil.
.