Try spraying WD40 in there till it just gushes out, to clean the mechanism. Then let it dry out and drip some real oil in it. WD40 really doesn't excel as a lube; it wears off too quick. But it's great for getting things going.
The best lube for locks is supposed to be graphite, but that's a dry lube, and I'm not sure how it would work if applied right after the WD. If anyone has any advice about that, I'd be interested, too.
Lastly, if it's old enough that it has a round key, just replace it before somebody pics it with a Bic pen.