WD-40 works great on loosening rusted material, but be mindful of where you apply it, especially on plastics. Their FAQ says don't put it on polycarbonate plastic, which is used for the transparent parts like windscreen, light pipes.

http://www.wd40.com.au/wd-40_faqs

http://transformers.wikia.com/wiki/Plastic

I'd suggest spraying or even saturating a cotton bud, then carefully rubbing it on the screw, with the aim of having some penetrate down into the thread. Maybe try it out on a piece that you're willing to write off.