Synthetic Oil - Do you use it in Cummins Engine?

Your eng should start easily down to -40 with grid heaters - about -30 without them so syn oil isn't really needed.
You also have an eng that will go 700,000 to well over 1 million miles with normal maint. So it isn't really needed for longevity either.
I tried in in two Cummins I owned but neither one got any better mileage or showed any kind of change.
But if your the type that just likes to use the very best to take care of his toys (and I sure repect that!) then go for it. It sure wont hurt.