Can I use graphite in a automobile ignition lock?
Let's see... should you inject a crystaline powder into an enclosed space that has only one access port? After numerous repairs on locks that have been filled with graphite by people "fixing" them, my professional locksmith opinion is NO. Do not put graphite into ANY lock. Besides filling the available space needed by the key, springs and "tumblers," the sharp edges of the graphite crystals will eventually wear down the brass pins. Sharp edges? Well, when you measure things in thousandths of an inch, graphite seems very large. I don't know how many times I have been called out to fix a lock because the key is hard (if not impossible) to insert, only to find that the farthest reaches of the keyhole has 1/8" of graphite blocking it.
So, should you use oil? Absolutely NOT! Oil becomes gummy, attracts and hols dirt, and will mess up your lock, too.
A silicon based lubricant, or WD40, or similar products work much better. They have the advantage of flowing to fill and lubricate the tiny places and then flowing out or evaporating. If your key still has trouble turning, it is usually because the key is poorly cut, and the "tumblers" will not align with the sheer line of the lock. Your best solution is probably to have a locksmith look at the lock and make a key to factory specifications. Remember, just like you cannot read a photocopy of a fax of a scan of a mimeographed page very easily, the lock cannot "read" a copy of a copy of a copy of a copy of the original key.
I just saw Dave's answer after finishing my post and that is 2 security professionals both on the same page.
The consumer would benefit from remembering that there are business's like ours that "specialize" in security and that is the only thing they do. We are not a Jack of all trades who know a little about a lot. However, pick the right locksmith with the right credentials as there are a lot of scam locksmiths.