Is it engine oil, or transmission fluid? The transmission oil cooler is in the radiator, and if it has a leak, the cooler pressure is higher than the cooling system pressure, and forces oil into the radiator. The same applies if it has an engine oil cooler in the radiator, not all vehicles do. Another possibility is a leaking head gasket, or a crack in the block or head, it would have to leak in a specific location near a pressurized oil passage.
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Friday, July 1st, 2011 AT 3:21 AM