If it is an automatic ( make and model omitted in question) there is every chance that it is transmission oil from a burst cooler in the radiator tank. The fix is a new radiator or and air to air cooler core fitted in front of the radiator. If the engine is a diesel or an engine that has a sump oil cooler then the oil core has burst allowing engine oil into the radiator vial the coolant system. It may be from a failing head gasket where the oil hole to the rocker gear is leaking across to a water jacket
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If it is an automatic ( make and model omitted in question) there is every chance that it is transmission oil from a burst cooler in the radiator tank. The fix is a new radiator or and air to air cooler core fitted in front of the radiator. If the engine is a diesel or an engine that has a sump oil cooler then the oil core has burst allowing engine oil into the radiator vial the coolant system. It may be from a failing head gasket where the oil hole to the rocker gear is leaking across to a water jacket