I've been doing refinishing including weathering and patinas for years and have learned to watch out if using WD-40 on metal parts.
It works by "displacing moisture" (great for the insides of distributor caps) but what it does is actually to absorb any water into the WD-40 itself. Now you have the liquid version of a wet rag. If left on the metal's surface, in time it will form rust. Guys spray their firearms down and put them away only to find that the guns, sitting in the gun safe, have still rusted over time.
Freeing up stuck or rusted parts? Kroil (Marvel Mystery Oil in an aerosol can) works well.
Powder solvents like Hoppe's #9; are good for removing fouling but not what you want staying on the finish.
Same precaution with rust removers or rust "converters". Since bluing and case hardening color finishes; are a carefully controlled rusting process but still rust all the same. Messing up beautiful bluing or color case hardening can happen if you're not clear on what you're putting on your firearms..
Better to use a good gun oil to protect metal surfaces.
If you have a relic and you want to preserve the original finish, weathering, and all; you might try Flood's Penetrol. It's concocted to help smooth out brush marks in painted finishes but used alone, it forms a nice protective coating that also stops further rusting. Works on wood too.
Wipe it on, wipe off the excess and let dry. The only drawback, if you want to call it that, is that it'll slightly darken the surfaces it's applied to.
