On high usage vehicles, the ignition lock may become difficult to turn due to wear of the ignition lock and tumblers. Replacement of the ignition lock cylinder will generally be required to correct this concern.Google+
Car Problem Reports
Mercedes-Benz 300SL High Usage Vehicles May Experience Ignition Lock and Tumbler Failures
Mercedes-Benz 300SL Problem
Average Mileage: 104,000 mi (67,000 mi - 141,000 mi)
Visitor, 1992 Mercedes-Benz 300SL, 67,000 mi
Key switch jams. Still not repaired
Very difficult to turn key. No lubrication helps. I will have to have the tumblers replaced soon.