The best penetrating oil of all is "oil of wintergreen". I can smell traces of it in MMO. OOWG is very expensive, used by deck hands on battleships to change out gun barrels. It works along with heat. A little goes a long way.
PB rust blaster is very good.......WD40 not as good, it seems to be more of a rust attractor.
A 50-50 mix of acetone and ATF is reported to work very well.
Hoppes gun cleaning solvent is very good, too.