The Ford Explorer might not have been the world's first SUV, and it might technically not have followed the original definition of an SUV; but to a whole generation of Americans, Ford's instant classic defined the genre. The Explorer was also one of the last to bear in its mighty bay another classic of its genre: the 5.0-liter Windsor engine. Even the base model's 4.0-liter had a good bit of provenance, dating all the way back to the the 2.0-liter version used in the 1964 Ford Taunus. With all that history behind this Explorer's chassis and engines, you can bet you're not the first from the Ford camp to find some oil in the radiator of one of the above.
It goes without saying that fluids don't mix -- but they do come close in a lot of places. When you find oil in your radiator, or coolant in your oil, the first question you need to ask yourself is where oil and water come close to meeting. The second question you need to ask is whether or not you're looking at engine oil. Unlike the same-year Expedition with the modular engine, which was notorious for leakages at the oil adapter gasket, the 5.0-liter and Cologne V-6 engines had no regular fluid crossover problems at this point in their life cycles.
You have two kinds of potential oil leakage on the table here: engine oil and transmission fluid, which can look almost identical when emulsified in dirty radiator water. To test, allow your truck to sit for a few hours, then slowly withdraw a quantity of coolant with a turkey baster. Squirt it into a clear, glass jar and let it sit for another hour at least. The oil will float to the top, at which point you can shine a light through the edge of the oil layer and look at the color. If red or reddish brown, it's transmission fluid. If that's the case, you have a leak in the transmission cooler inside your radiator, and need to replace the radiator. If any other color, it's likely engine oil.
The good news here is that since both engines had been in production for decades, Ford long since worked out their biggest bugs and they didn't have many opportunities for fluid crossovers. On these engines, you might find coolant in the oil after blowing an intake gasket, but oil in the coolant is very uncommon. And there's the bad news, because broken head bolts and blown head gaskets aren't particularly uncommon on 5.0-liter engines of this age. Head gasket failures aren't exactly unheard of on 4.0-liters, either; an engine block originally designed to accommodate 2.0 liters doesn't leave much gasket surface to spare after doubling the displacement.
Head gaskets can fail in many ways, and exhibit many different symptoms. Classic symptoms are rough running, misfires, loss of compression, exhaust gas puffing out of the crankcase through the oil fill port, and coolant in the engine oil. Yours may not have failed in a way that would exhibit these symptoms, though; it's possible that you could have a minor blowout that's sending pressurized oil into the coolant. If that's the case, you might consider using a "block sealing" additive in your coolant before tearing the engine apart and replacing the gasket. Some of the better additives can permanently seal a minor head gasket failure, particularly if it's a failure along a coolant passage. It's worth a shot before you tear the engine apart.
