300 SEL 6.3 Central Door Locks
 

The drivers side door lock is very hard to turn. When it does turn the locking system works well.

Is there a way to lubricate or adjust the mechanism to work more easily. I am afraid I am going to break the key off in the lock.
Thanks!

Have you taken the door panel off yet ?
Also, is the door fully closing and latching ?
If not, then the lock mechanism has to be forced down.
Sometimes this can be fixed with a door-striker alignment.
The sliding 3-way valve could be lubed, once you have the door panel off, double check the operation of all of the levers and look for binding. This is what happened to me, once I got the driver’s door working, all of the other door worked great.
Good luck !
Robert

On the 109 series as well as most other MB chassis, the lock assy can easily be removed by opening the door and removing the 1 or 2 screws at the back edge of the door. once the screw or screws are removed, slide the handle towards the front of the car about 10mm and pull it straight out. You may have to tap it a little if it hasn't been out for awhile. You can now spray lube to your hearts content. Don't just concentrate on the handle; spray the latch mechanism inside the door.

Good luck,
Peter

PS: Models after 1980 or so require you insert the key in the lock and turn to remove. Antitheft.

After you get the door handle off (without removing door panel) as Peter said, you should be able to tell if the resistance comes from the tumbler assy or inside the door at the latch.

The tumbler is relatively easy to diassemble and should be if the resistance is there. When the tumbler has been released from the handle (by removing the actuating lever and the clip, be sure and remove with the key installed. This keeps any free tumblers from springing or falling out (saves having to recode the tumbler on reassembly). In your case if the bind is the tumblers they will probably come out near frozen. each will have to be pried with a pick, out of the body. All should be cleaned, including the body and reassembled with light lubrication.