White paint in the kettle: If you don't mix the coolant with oily liquid, the oil should enter the cooling system, which means that the lubrication system of your engine is "in contact" with the cooling system, so you must go to the repair shop to check it to avoid serious problems. Of course, because your engine has been overhauled, it is inevitable that oil will enter the coolant during maintenance, which will also cause this situation. "50,000 kilometers" should be more than one year. I wonder if you have changed the coolant (antifreeze is usually changed once every two years). If it has been changed, it should be judged as "adultery". I suggest you see for yourself whether the oil is white or whether there is a large amount of white paste at the oil dipstick socket. If there is any further judgment, it is adultery.
Hazard: You know, insufficient engine oil or water in engine oil will lead to increased engine wear.