A non polar solvent will remove oils, varnishes, shellacs, lacquers, (all at different rates) without dissolving the metal oxides that make up case coloring.
Don't scrub.

You can rank their aggressiveness by volatility.
Hotter solvents evaporate faster.
So, acetone, ether, mek, lacquer thinner, xylol, pentane, hexane, heptane, petroleum distillates.

If you dab some on under the forend, and nothing lifts to a swab, it's the oxidation of the metal surface, not oxidized oils.

Off to the races.

Out there doing it best I can.