Not sure what Dodge uses, Cummins uses valvoline. Cummins has been using Valvo for a number of years and most Cummins dealers use Valvo as well. You'll find either Prem Blue (which if 15w40) or the Prem Blue Extreme that is 5w40. The engines are usually shipped dry from Cummins and filled at the point of installation. I know that every engine that left the KW factory in Washington had Delo 400 in it for a number of years. I haven't looked lately so I'm not sure if KW is still using Delo or not.
The 6B engine family is very easy on oil. Any decent quality dino diesel oil will work fine. For better cold start performance and stability the PBE 5w40/RTS 5W40/S9000 5W40 are all excellent oils for the money and work great in the B's.