I have recently noticed that the coolant in my 1985 190e is almost black. I flushed the system and since then I have noticed the coolant has looked black almost like oil is in the system. Is there a way to test if it is oil and to find out where it is coming from? (From lines for transmission into radiator) There does not seem to be any coolant in oil or transmission fluid. Also, is there a limited slip differential available from another model that will fit my car?