what car do you have?
the radiator should be filled up from the highest point which is usuallyy the radiator cap BUT the Mazda Astinia 323 are filled from on top of the engine as their radiators are situated below the motor and hence the waters highest point is not at the radiator...
i do not see an issue using the engine oil flush as long as it is in the engine and not the radiator
cheers...
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