An interesting video, very informational

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.