... even if the car still shuts down, I vote that this is a classic symptom of a failed fuel cut-off valve that has only a small has a hole in diaphragm and this is allows the oil to be sucked up from the IP's lubrication and finds it's way out and drips from the old worn out rubber fittings on the ignition switch when the engine is shut off and vacuum is at zero. This happened on my 240D long before it got bad enough to cause the engine to be slow in shutting down. Stick a vacuum pump/gauge on the vacuum fuel cut-off valve on the back of the IP and you will see that it will not hold a vacuum as it should.

Sam