1995 E320 OIL IN COOLANT TANK

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I was checking over the car under the hood , I have had it for 4 weeks , everything is nice and clean , the car has been well cared for ,the coolant overflow tank was not very clear and I dropped a rag into the coolant and there was an obvious oil on top, the ar runs perfect and I was wondering if there is anywhere else the oil could come from other than the headgasket any ideas thanks