Have a 1998 280 c Mercedes . Have some kind of oil in antifreeze overflow tank.
IF it is dark color oil, it might be transmission fluid mixed with your anti freeze. there is a transmission cooling system within the radiator and it seems to be leaking within the radiator.
personally would flush the coolant system (drain, flush, refill). if it happens again there has to be a leak. check your oil levels on transmitions to see if it drops. You may just have to replace some gaskets. but the diagnostic is good to start with
I had that done, came back and is a brown color like motor oil. Going to have a leak down test done next. Thanks
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