the engine runs great but the oil & coolant are mixing & pouring into the reserve bottle but not in the engine.
coolant & motor oil mixing in radiator on 1999 Ford E-250
I just looked at the workshop manual on the 4.6 and 5.4L engine both have an engine oil cooler on the side of the engine (at oil filter) with coolant hoses attached to it. Perhaps the oil cooler is leaking and allowing the transfer of oil into the cooling system. This problem is only going to get worse and needs to be fixed immediately. The cooling system will have to be thoroughly flushed out as oil will damage rubber coolant hoses.
after you install the new oil cooler , does it make a difference which hose from the radiator goes to which side of the oil cooler ?
Looks like you have a bad oil cooler. Replace the cooler, flush the entire cooling system - you might need to do it more than once - replace the missing oil and that should be it.
I have the same problem , I replaced the oil cooler, thermostat , flushed 4 times with water but still overheats ?
i agree w/ other responses, failed oil cooler. easy enough to replace.