To have oil in the coolant suggests a cylinder head gasket is leaking but considering the cooling system stays near 15 pounds of pressure, the oil would have to be leaking from a pressurized passage in the engine. When the engine is stopped but still hot, pressurized coolant would leak into the oil, so that is not a likely suspect.
By "air cooling", I assume you mean you have a separate transmission cooler in front of the radiator. You do indeed have a transmission cooler built into the radiator too but someone may have bypassed that if they put the external cooler on as an accessory. Normally both are used when an external cooler is added.
Also look for an oil cooler at the base of the filter. Some engines have an adapter, about 2" wide, where coolant circulates through to draw heat from the oil. I've heard of those leaking externally but not mixing of the fluids.
My recommendation is to add a small bottle of dark purple dye to the oil, then check a few days later with a black light to see where it went. The dye will show up as a bright yellow stain that you can follow back to the source. If the oil was going into the coolant, with as much as you say you're losing, you'd have mud in the radiator, not just a film. It's more likely the oil is being burned in the engine and you'll find dye inside the tail pipe.
Auto parts stores will have the dye, and those that rent or borrow tools should have a black light.
Sunday, July 6th, 2014 AT 9:51 PM