I have a Ford F150, 2004, 5.4 supercab. The engine has the terminal problem where oil is leaking into the fuel system. My mechanic, who has given me excellent care of this vehicle recommends not putting any more money into this vehicle. What is it worth in this condition?

You should Check to see if the PCM is storing codes before making any decisions. I would also recommend a second opinion. Other then that, "does the truck run rough like it's missing at a certain RPM or speed?" Sometimes the coil packs go bad and you may "not" be getting fire to a cylinder causing gasoline to seep into the oil. How does the exhaust fumes look? No color? Smell gas? White smoke? Black smoke? When checking for codes it will let you know if a pack is bad and which one it is, "if that is the case." Another idea is to run a compression test on each cylinder. How many miles do you have on the truck? Check for Vacuum leaks. Clean the Mass Air sensor. Although, "I think a code test would be the best to start with."

Are you sure oil is getting into fuel? Or fuel into the oil? Because it's not really possible for oil to find its way to your fuel

Just my two cents. If the vehicle is in good to great condition why junk it. My oldest is a 1996 which has been meticulously maintained from the day I purchased it, even the body and interior. Handed it down to my son in 2003 who has driven it and maintained it's appearance. The engine finally went. He spent 4G's having a manufactured engine installed. His reasoning was it was the engine cost about two G's more than the sales tax on the new truck he was looking at. Anyway, your market will be limited to those willing to purchase the truck then replace/rebuild the original engine.