Main cause of problems in the 6.0
Ford decided to use crappy coolant. Add into the fact that there is plenty of casting sand left in the block and heads, and its a recipe for disaster. In the 6.0 water goes through the oil cooler (liquid to liquid cooler, coolant is used to cool the engine oil). Then after it goes through the oil cooler, it goes through the EGR cooler.
Because of all the casting sand, and other things like rust, the oil cooler slowly clogs up. This in itself is not that big of a deal, as long as oil temps dont get to far out of hand. What happens is they clog so much the EGR cooler gets less coolant than it needs. So now you got exhaust heating the EGR system so much it causes the little coolant that is going through it to flash boil. The steam then rupture the EGR cooler. Now you have steam/coolant getting into the intake, and the engine starts to burn it. Eventually you blow a head gasket.
In short, oil cooler clogs, EGR system boils, steam into intake, blown head gasket.
To fix this you need to thouroughly flush the cooling system and switch to an ELC type coolant. Instal a coolant filter, to help trap all the debris in the system. EGR delete should also be done, or an upgraded EGR cooler. Lastly, change your oil cooler before it gets to clogged up.