Cummins does not recommend synthetic but traditional 15w40 oil on their diesel engines, unless one is in a cold weather. Their recommended oil is Valvoline Premium Blue and the filter is Fleetguard. I always use these two.
Had a chance talking with a Cummins engineer at their lubrication division. For recreational vehicle application, change oil and filter every other year, but not to exceed 15,000 miles.
YMMV.