I frequently encounter hard to remove screws when dismantling motorcycles. A buddy of mine in Mesa, AZ who is an aircraft mechanic recommended a 50/50 mixture of acetone and anutomatic transmission fluid to help loosen the most difficult ones. The acetone thins the ATF to a point that there is a capillary suction phenomenon that draws the mixture into the threads. ATF, when allowed to sit for a while, will work wonders on rust & corrosion. I typically apply it and let it sit for a day or two before taking a hand-held impact to the problem. I rarely have to give one a second dose of the mixture to get the job done.