Brand new keys are hard to turn in door handle lock
 

I ordered two brand new keys from the MB dealer for the 81 300SD and gave them a try today. The keys work just fine in the ignition, glove box lock, and trunk lock, but not the door handles. With the door handles, the new key won't turn unless you pull the key back ever so slightly, and then there's some resistance when pulling the key out. The old key works just fine. The tumblers are original to the door lock as far as I know. My guess is that the new key + the old tumbler = iffy operation. Maybe I need to use a lubricant like graphite powder to make things go smoothly. Thoughts?

no graphite; it gunks up. use a PTFE (dry lubricant) after cleaning out the lock cylinders, i like to use the PB blaster creeping oil stuff to clean as it breaks up most of the gunk. you could just spray it in there and let it work for a little or take the whole lock cylinder out and soak it in the PB stuff (the door handles come off rather easy, then just knock out the pin holding in the lock cylinder)

Spray some air first into it maybe some dust has accumulated. Safer than spraying a liquid lubricant (which will collect any dust to begin with). Graphite is a dry lubricant so not much of a problem with this.

Also make sure the "wall" of the tumbler exterior (where the key head rests against when inserted) is not caved in. Sometimes through usage this happens. Does the tumbler door work properly?

My 82 SD is like this. put the key in, pull out slightly, and it works smoothly. my original key is EVER SO SLIGHTLY different from the new one.

Same here. I compared the new and old key. The cuts on the old key and the new key correspond, but the new key's cuts are sharper.