My husband uses a good penetrating oil to loosen stuck screws. I'm not a sewing machine mechanic so don't know if this is what you should be doing. I'd think about using a Kroil product as recommended by this website
http://www.mckennalinn.com/cleaning-...-machines.html
I've gotten good deals on Aero Kroil from this website.
http://www.kanolabs.com/msn/