Jennifer in Moose Jaw set in her tip that she unsticks door locks that don't want to work by using a lead pencil. She rubs the led right on her key, and lets the key carry the graphite like lead right into the lock.
This got us started, so we surveyed some locksmiths to see what they thought about the tip. They weren't against it, no harm done with the lead. But we did learn a lot. The worst jammer of open locks, like door locks that have air blowing in and out of them, is dust. They basically don't want any lubricant at all in such locks. Oil would attract dust immediately. Silicone builds up a layer, and these close fitting locks with tiny springs can resist much build-up of anything. Even graphite is reserved for closed locks that don't get air blowing through them, while white lithium grease is used for large mechanisms like vaults.
What to do with the front door lock? They spray it liberally with WD-40. Why this lubricant and not others. Well actually WD-40 has only recently put lubricant in some of their cans, the ones that say "with added lubricant". Regular WD-40 works as a cleaner but not as a lubricant. Have you not noticed squeaky hinges "lubricated" with WD-40 don't stay quiet very long? They have only begun to add lubricant to it because the public began to realise that other sprays were doing a far better job of lubricating. No, WD-40 was actually designed as a Water Displacement spray, not a lubricant. But somehow the marketing guys got us all spraying it over and over on squeaky things. There are many other products on the market that do a far better job of lubricating than WD-40.
Check out OVERVIEW- Lubricants - Why are there so many choices?