[emeritus]

Petrol lighter fluid or Plus-gas if you can find some, is far more penetrating than ordinary WD40, although I understand that a proper penetrating oil is available under the same or a similar brand name. I have used ordinary 4 star in an emergency, but the lighter fluid does not have the noxious additives you get with motor spirit. Don't expect to be able to unscrew immediately - steel is not infinitely rigid and you will have a better chance of success by firmly attempting to rotate the screw to and fro to initially loosen the thread nearest the surface while the deeper portion remains fixed. To and fro movement should start to pump out the corrosion and allow the penetrating fluid to progressively penetrate deeper. Patience will probably be required.