Oil is leaking and mixing in with the coolant in the reservoir. No warning lights are on, no overheating, and the car performs fine.
If you are seeing oil in the water it could indicate a blown head gasket and should be looked at by a technician. I would not drive the car until you fix the problem because a blown head gasket could allow water into the motor and cause a number of problems. None which are good or end with the motor running again.
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