Locks, like other mechanical systems with moving parts, need to be lubricated to increase their lifespan of trouble-free functioning. If you don’t lubricate your lock, it will become fussy after some years. If you have trouble with your lock not turning even though you or your husband or wife has been liberally coating the inside of your lock with graphite powder for years, the source of your troubles could actually be because of the graphite and not in spite of it.
I see the inside of locks all the time. I see a lot of them every day, seven days a week. I can tell immediately when somebody has been using graphite. It gets all over my fingers. If I am rekeying the cylinder, it is usually really hard to knock the old pins out of the cylinder, because the graphite has formed a sort of paste around the pins.
That paste translates to trouble when you try to use a key with the same setup. That paste makes it hard for me to move the pins inside the cylinder, and it makes it hard for a key to move them. Rather, once the key moves past them the paste prevents the springs above the pins from pushing the pins down into the valleys of the key. If that doesn’t happen, the lock’s cylinder plug won’t turn!
I have even run into locks where some old guy has been putting graphite in a lock for so many years that a cake of graphite has built up in the back of the lock and is so big that it obstructs the key from going in to the lock all the way, and of course if you can’t put the key all the way in your lock you can’t unlock it.
Long story short, don’t use graphite, especially outdoors. Use WD-40 or something else like Tri-Flow. In Seattle, these lubricants push moisture out of the lock and don’t form a paste like graphite. In hotter climates they can form a gummy film. If you want to use the best possible lubricant, use some powdered Teflon from Dupont. That works a charm in all climes. It’s just really expensive!