Stuck passenger door lock (1998 Mercury Grand Marquis LS)

Since the keyless entry system stopped working last year, I've had to use the old-fashioned keyed locks, and number pad, to access my 1998 Mercury Grand Marquis.

Driver door and trunk locks work flawlessly, but the passenger door lock is uncooperative. It seems as if it only works when the temperature is somewhat warm - let's say, above 50°F. Which it is here only sometimes in the afternoon, this being winter in my area.

It goes like this: I'll insert the key, attempt to turn it...but, it doesn't. Maybe wiggles a little. That's it. I'm not going to force it, since I don't want to break the key off in the lock, and it probably still wouldn't turn.

At least part of the locking mechanism does function. I can unlock and lock the passenger door from either the driver's side number pad, or door-mounted lock/unlock switches (both driver and passenger side). It's only directly turning the lock with the key which doesn't work.

I've tried rattling the key in the lock. No good.

I've tried my extra key. No difference.

I've squirted some graphite lube in the keyhole. That helped when the lock stopped wanting to turn a couple of years ago. This time, it didn't help.

I'm reluctant to put anything else in the keyhole. Someone suggested WD-40, but that seems like a terrible idea, since it's not actually a lubricant. 3-in-1 or other oil could gunk up something sensitive.

Ours is a very dry climate, so I doubt anything is binding up due to rust anyway.

Short of taking this to a mechanic, any ideas?

How much of a project would it be to swap out the lock myself? Am I going to run into headaches due to the vehicle's notorious first-generation PATS?

It's weird that the *passenger* door lock has seemingly worn out, since it's the driver side lock that gets used most often.