Oil in a car's radiator can be caused by several different issues. In some cases, an engine head gasket failure or a cracked engine block can cause oil to enter the cooling system and mix with the coolant. Other causes can include a leaking oil cooler, an oil filter that is clogged or has become loose, or an overly full crankcase. In any case, the oil should be drained from the radiator, the issue causing the leak should be addressed, and the radiator should be refilled with the correct coolant and oil mixture. Failing to address the issue can result in engine damage, overheating, and a decrease in engine performance.