I don't agree with Ox's info. Can't see how an upper intake that has no oil in it could contaminate it. Upper gaskets have no oil, coolant elbows have nothing to do with oil. Even lower manifold gaskets have no oil.
What color does the oil appear to be?
Has the cooling system been maintained properly?
If the oil is reddish in color, you have a trans cooler leak. The radiator would need to be replaced or the trans cooler in it needs to be bypassed and plugged.
Well if someone mixed green coolant with dexcool, it will turn brown/black.