You have a high pressure oil leak, it could be @ the hp pump or block off tubes. Once your oil gets hot it is too thin and will blow by the oring(s) that are worn out. You need 600psi of oil pressure in order for your injection system to work. It will get worse and will not start even when cold. If your truck is an 05 + you should replace the HPOP (high pressure oil pump) they fail all the time. Also replace the EGR cooler with a bullet proof diesel one while it is apart, it will save you $800 in labor because it will fail! I own a diesel shop and I specialize in Powersrtokes. Avoid the dealership unless it is under warranty.