+1. The first time I used Kroil was for a stuck plug screw just like the OP's. It wouldn't budge. Soaked it in Kroil and literally spun it out the next morning.Kroil? Supposed to penetrate areas down to 1/1,000,000 of an inch.
Kroil works better than any other penetrant I've ever tried. I gave my Liquid Wrench away.