Two other products that are more available in the US are PB Blaster and Kroil. You did not mention your frame material. I will assume it is metal. Spraying this on the outside may help you out. I think your best bet would be to remove the bottom bracket and spray the oil on the other side of the screw. This will get the fluid into the threads.

If the screw extends into the bottom bracket I would grind off the rusty bit that sticks out.

If you have stripped the head of the screw you may have to use vice grips to grab the head. If that fails you may need to drill out the screw. Start with a small drill perfectly centered and work up very slowly. Hope this helps.

Ride Safe,

Joe