My dad has a 2004 6.0L with 268K miles on it. Engine died on road. Was towed to shop, where it was determined that the hpop failed. The hpop was replaced, along with the regulator. Now, with almost 4K miles since then, it is still running rough, like running on 4 cylinders, for maybe 10 minutes of driving. Then it starts to runs normal. The mechanic that did the work said it may take up to 5K miles of driving for it to clear out. He told my dad about a product to put in the oil, it comes in a 2qt bottle and costs $30. Anyone have any ideas as to why it does this?