Brand new keys are hard to turn in door handle lock

[H-townbenzoboy]

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?