Graphite is the best lubricant for the W123 ignition switch. Using a number 2 pencil with graphite lead rub the pencil lead on your key grooves on a regular basis. Also be sure your key chain has very few keys. It has been documented that excessive weight on a W123 key chain will cause problems over time for the ignition switch. Also, make sure your key is made of steel and not brass. The brass key will over time leave brass filings in the switch which are harmful to the switch.