If it may help -
Got 2 spare keys in addition, when I bought my 190D, 9 years ago.
Immediately started using the one which worked the smoothest.
I went to a keysmith and brought the smoothest going key along with a Silca HU64P (works with my '91 190D) to make at least one spare.
Also, even the best key always went a bit notchy in the drivers side door obviously because of the most usage. I took the key tumbler out of door, careful not to loose any micro springs(!!!), inserted key and then ground/dremel'd down to flush with tumbler. AFAIK, the locking pins needing a very slight shave, was the 3 outermost, out of a total of 10. Added regular lubrication of tumblers to schedule. The steering tumbler is somewhat notchy although never jamming. Did my research and contingencies about this.