I have a 1987 300D which is auto and has the OM603 engine. The rear crank seal is starting to go as I have noticed a few drops of oil on the garage floor and it is engine oil coming from between the engine and gearbox. How long should I leave this as there is not a lot of oil, but it can only get worse! Since it is auto, there is no danger of oil on the clutch plates. Thanks.