on the engines you're probably familiar with, oil is for lubrication and you'd be correct.
In the 6.0L the oil is also used for actuating the fuel injectors, and in order to do that it's pumped up to very high pressure.
It means the oil needs to be able to put up with abuse not typical in other engines.
Residue can build up that needs to be removed by an added chemical in order to let the injectors function properly.
There are lots of threads with jawing about oil, but I've seen a couple that were pretty useful about the properties you need and that are attacked by this engine.
Bottom line is, to me, synthetic oil is good most of the time, but the buildup does require the occasional addition of RevX.