All locks have moving parts inside the key way. So if your key goes into the keyhole rough or turns with difficulty most likely the problem is that it is dried out. Most locks require lubricant about every 3 months or so on average. But if the lock is exposed to the sun or rain it may need to be oiled more often. Basically when it starts to get rough inserting the key or turning the key, it is time to lubricate it. Now as for what lubricant to use? Well if you live in mostly warm weather as I do here in the southern U.S. you will want to use some sort of spray lubricant such as WD-40 or any silicone spray lubricant is fine. But if you live in a very cold climate you will want to use the old standby of graphite because it can withstand very low temperatures without freezing.
But you should NEVER use grease or any heavy type oils because they collect dirt and can gum up the lock very easily.