What do you mean by mixing oil? If you mean by using different weights or synthetic vs conventional then just drain and fill with what you want to use. No reason to flush.
If you have oil in the coolant then a flush would be needed after the head gasket is repaired.
I ACCIDENTLY PUT MOTER OIL IN THE COOLANT. WHAT WILL THIS DO TO MY CAR? iT'S A 96 CHEVY CORISICA.
It will probably make your car overheat but no big deal. Flush it once or better yet twice and then install new 50/50 coolant.
Your hoses will start to swell over time and get soft if you put oil in the coolant
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