Oil in the coolant resevior

[mr benz]

I own a 190e 2.3 1987
Recently i have notice oil going into my coolant resevior. The cap is gooky.
I believe that the coolant is mixing with oil but i cant detect where or how this problem is occuing, and how it's geting into the coolant line and into the resevior.

Please help and advise.

Thank you.