Oil going into the plastic coolant container. on 1993 Mercedes-Benz 300SE

I had my water pump replaced, oil is going into the plastic overflow container, no antifreeze in the engine though, the car never over heated could it be a head gasket? the car runs does'nt over heat or smoke, it;s just that i have to keep putting oil in it!

Asked by for the 1993 Mercedes-Benz 300SE
If the coolant is disappearing with no visual leaks, then there are 2 possibilities. The coolant is leaking from the heater core inside the dash, you would smell coolant inside the car and have wet carpets though. The other is coolant is getting into the combustion chambers via a cracked head/block or leaking head gasket.
Since you have oil in the coolant and your losing coolant, I think it's the headgasket block/head crack.
To check for these, please look at this article:
1 more answer
The head gasket should not apply to this problem. More than likely its the valley pan gasket. It seperates the water coolant from the oil.