Finding oil in the coolant reservoir of a car radiator means that one of the main gaskets in the engine has failed. This means that a leak has formed in the engine that could ruin it quickly. The best thing to do is have it towed to a mechanic.
When oil and coolant mix, the combination is not healthy for the engine, as it has the texture of a slushy drink or milkshake. The most frequent way that oil ends up in the reservoir is a failure of a head gasket. Another possibility is the formation of a crack within a cylinder head in between the coolant passages and oil galleys inside the engine. An engine block can also develop a crack that allows oil to pass between the two areas.
Once a trained mechanic has the car, he looks to see if the head gasket is the problem. About 90 percent of the time, the head gasket is the culprit. Getting the car to a mechanic before the fluids ruin the engine saves the motor, which the mechanic can drain and repair. If a car owner notices this sort of fluid in the coolant reservoir, he should not drive it until it has been fixed.