I've had excellent results with Birchwood-Casey's Sheath, even on guns used in small boats in saltwater. I usually see it in a spray can, but it also comes in lighter-fluid-size can. In addition, it dries and doesn't get all over everything like oil, grease and silicone does. Easier, but more expensive, are the treated plastic storage bags from Bianchi and Brownell's.
Having said all of that, I got along fine for 35 yrs. here in beautiful, green (read humid) Virginia will just number 9, gun oil and maybe a silicone cloth for fingerprints. Just don't leave your gun in a damp case. John
P.S. - Richmond weather now: foggy, 57 and 100% humidity.