WD40 is a good cleaner and can free up stuck parts sometimes, but it isn't much of a lube. Once it dries, it doesn't leave much, if any, lube and protection behind. Parts often rust up faster than before after using WD40 on them. You would probably be better off with a penetrating oil of some sort, followed by some regular oil after the parts free up. But honestly, the proper way to do it is to take it apart. Then you can clean all the parts and properly lube them. I use white lithium grease for the sliding plates in the lock cylinder, a little normal grease for all the other moving parts.