Any locksmith who recommends WD-40 is REALLY just looking for future business. This is the worst possible substance you can use on a moving part. It is designed so that the lubricating portion will offgas in a month or two leaving behind a gummy tar like substance. Fine for when you are wanting to waterproof something but horrible for moving parts.

At this point then people spray in more WD-40. The volatiles will temporarily dissove the tar, allowing the part to move. But when they evaporate you now have twice as much gum. After a year or two the lock is useless.

The best substance is plain light grade oil. But it will run over everything so keep napkins handy. Graphite is good for certain installations. It won't "dry out" (it is a powder, not a liquid) but does not have the rust prevention of oil.

There are plenty of locksmiths who don't know their rear end from a hole in the ground as you can see from reading the above posts.