It's possible for fluids to mix but that is by far the least common symptom. There's oil draining back under no pressure, oil being pumped to the valve train under about 40 pounds of pressure, coolant under 15 pounds of pressure, and combustion gases under hundreds of pounds of pressure intermittently. Antifreeze becomes acidic due to the normal seepage of combustion gases into it. It has a number of additives including corrosion inhibitors that wear out in about two years. Because of that acid, and the high pressure in the cooling system, that is where most corrosion and leaks occur. It's not common for any oil passages to be involved with that leakage.
When there is a leak between the cooling and oiling systems, which fluid goes where depends on a number of things. Oil at 40 pounds of pressure might get pushed into the cooling system, but when the engine is stopped and still hot, the coolant will stay at 15 pounds for a minute or two, and that may not be long enough or high enough pressure to push coolant into the oil.
Monday, December 9th, 2013 AT 9:44 PM