Once more, a 10w-30 is what you want to use in this engine, there isn't any need for a synthetic. I have over 350,000 mi. In a 6.0, and UOA data from every OC that we're run out to 7,500 mi. Intervals to prove my point.
There were multiple times cold soak starts happened at -40, no issues, no injectors ever replaced, and stellar UOA's from 10w-30, including loaded operation during summer at 90+f running 150 mi. Down the highway. The OP can assume all he wants he "needs" a synthetic lube and change it at 5,000 mi intervals to make him feel better, but the 6.0 won't know the difference.