WD 40 will work-Short term. Long term, any oils will evaporate the carrier and leave thick, dust catching oil in the lock and on the pins and tumbler causing grief and angst. Most of our cars could use a quick lock tune up: Take off outer handle. It takes all of 45 second to a minute on 123 or 126 Chassis cars. Clean lock assembly thoroughly with electrical contact cleaner. Later 126's have a electrical switch on them also. After cleaning, lube the lock and mechanism with Powered Graphite. Work the key in and out and get that graphite down in all the pins and springs. Graphite is available from any hardware store or Pep Boys. Your locks will work like the day they came off the line in Germany!!

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Orland Park, IL
1985 300SD 215,000 miles