Kroil works well too.
http://www.kanolabs.com/I used it on my Stoner leg levelers. They were rusted solid. Worked well.
I used it on my Vendo Ice Cream machine plastic door. Was not a good idea. Although the rusted screw broke free, it had a capillary effect on a paper insert and distributed throughout my plastic window and ruined it. It semi changed the plastics state. Great on metal, just stay away from plastic parts.