1998 Ford Mustang GT (startup after 9 years) The engine started right up, and as I watched the engine after it had started, Light brown oil (obviously mixed with coolant) started to flow very quickly out of my resivoir. Also, I found that the oil and coolant mix was found everywhere in the cooling system including the thermostat, but it was not in the oil pan when i drained it. after doing research, most people said it was the head gasket. I found that difficult to believe because of how fast the oil was spewing out of my coolant resivoir. further research told me it could be the oil cooler gasket... I took the oil cooler out (water and oil mixed were inside the oil cooler) but the adapter gasket was in perfect condition. No faults or anything. I decided to test my oil cooler by sealing one side off to a block of wood, and putting the water in where the coolant would usually go. I did this and water began to flow out of the oil cooler where the oil filter would be if it was connected. Is that suppose to happen? Is it possible for there to be a fault or internat crack inside the oil cooler allowing them to mix? are there aftermarket solutions or do i pay $400 for a brand new oil cooler?