I recently made the Yale lock on my front door much easier to open by spraying some WD-40 in there.
However, I just saw this question on WD-40 not being a “true” lubricant, and wondered whether it was appropriate to use on Yale locks?
OLD ANSWER (Improved below)
Graphite powder is the preferred lubricant for locks. You should be able to get it at any hardware store in a squeeze bottle that is half air, allowing you to blow it right into the keyway. You are going to have to wait a while before putting it on, as the residual WD-40 will gum it up.
EDIT: As per MrSquonk's comment below -- Try coating the key and work it in slowly. It's less messy.
LATE EDIT: Comment below edited into the answer. Please read full comment thread.
Yale USA says:
Yale® KeyMark® cylinders are lubricated from the factory with a Teflon® lubrication. Cylinders should be lubricated periodically depending upon environmental conditions and usage. LAB Lube is the approved lubricant. Caution: It is not recommended to lubricate cylinders with oil or to mix lubricants. "
Micronized polytetrafluoroethylene (PTFE) Powder
A dry white powder lubricant that will not "cake-up" as graphite does.
For locks that are in such bad shape that one is unable to get graphite powder in it, you can first use a little bit of LPS 1 (Greaseless lubricant). Do not use any kind of silicone lube, that's FAR worse than WD-40 (I had a guy come in with a couple locks he'd done that to, it ended up costing him a good bit of labor for me to undo that mess)!
This page on YaleDoor.co.uk
Says the opposite ... "Any, “all purpose” oil or lubrication will do the job, but be sure never to lubricate your door locks with powder graphite, as it will do more harm than good. Simply insert the straw (which is normally supplied with lubricants such as WD40) into the lock cylinder and spray away! "
Now I'm confused