None of my keys will completely slide into the lock on the passenger door of my 300SD. The driver's door and trunk locks work fine. On the passenger side the key will slide about 3/4 in, but no further. This makes it quite difficult to be a gentleman and open the door for my passengers. I would prefer not to replace the lock cylinder, and by that I mean if that is what I have to do it will remain unfixed. Any suggestions?

Lubricate?
 

I don't know if you've tried it already, but it's worth a try.

What do you suggest lubricating with? I have been hesitant to spray stuff into my door. I thought about using WD40, but I don't want it to seep past the cylinder and onto the back side of my interior door panel.

I heard that you shouldn't use WD-40.
But I have been using this stuff (on antenna, wiper, etc) and it seems to work well... it lubricates nicely and does not attract dirt. I believe it's called Teflon or something along those lines. But it is a spray. Just use it sparingly I guess? Or I heard someone used 3-in-1 household oil once and it worked well. I am sure a little will go a long way. Good luck.

I've used a spray called "Lock-ease" that is similar to WD-40 but contains graphite also. The lubricant in WD-40 is notoriously short-lived. It's really more of a water displacer and penetrating lubricant than a long-lasting high-quality lubricant. I'd be less afraid of Lock-ease than using an actual oil in my lock cylinders.

If the key isn't going in, I suspect that the lock tumblers are jammed or otherwise messed up.

The dealer can order you a new door handle for about $120 and you get a new key. Give them your VIN.

My 1983 SD needed the driver's door done, so it was less a matter of chivalry and more of a pain in the butt to get into the car.

Hope this helps,
Bob