Clarence M. Bates, of .45-70 revolver fame, introduced me to "100% PURE, NATURAL, wintergreen oil."
It is one of the greatest penetrants I have ever used. You get it at your local health food store (NOT Walgreen's et. al.) for about $3.00-4.00 for a small bottle. You don't need a lot of it, however, as it seeps right into any crevice, and generally a few drops are all you need.
Give it a try. I have gotten more old, rusty, corroded screws out using it, as well as barrels from old Italian Vetterli rifles.