Any good gun oil after it has been cleaned properly...
+1
If the gun is not properly cleaned nothing is going to properly protect the bore. If my gun is going to be fired within a few weeks or a few months it gets Rem Oil. Those guns that are more or less in permanent storage get Militec 1.
My guns have been left unattended in their safes for longer than two years at a time when i worked overseas. No gun ever rusted.