The best product I've found for cleaning oil from the cooling system is "Shout " laundry detergent. Sometimes the best thing is to replace the coolant expansion tank after flushing the system because it is very difficult to get clean. Also be sure to clean the coolant overflow tank inside the right fender if the car is equipped with one. If the car is driven for any length of time with oil in the system you will have to replace all coolant hoses because the oil makes them deteriorate terribly.

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Mercedes Benz Master Tech - Retired