I'm sure there are other penetrating oils that work as well, if not better. But, I use WD-40, followed by a few good whacks with a hammer. If it still won't budge, then use heat. You only need to heat metal up to about 300 degrees for it to expand. If you can get it to turn even a fraction, turn it the other way immediately. Then back, then in, back, etc. More penetrating oil, and work it back and forth, firmly but carefully. If it doesn't break off in the first half-turn, you got it made.