Valvoline Royal Purple 15W-40 Oil is the recommend product for Cummins engines. The oil containers even have the Cummins logo on the front label. I can usually find this on sale between $10.60 TO $11.50 a gallon depending on which store has the oil on sale. It is also recommend that you change your oil filter at every oil change.
Please do not buy the cheap oil filters that the parts stores sell, buy the correct Fleetguard filters for your engine. Your engine will thank you in the long run. The Fleetguard filters are engineered for the Cummins engines. The filters provide the correct filter media, inlet/outlet hole pattern, Belleville spring washer and bypass to name a few items that will provide the proper oil filtration and circulation of the oil to your engine.
2016 34RL CC; 2008 Ram Mega Cab 2500HD, 6.7L, 68RFE 6 speed, 4X4, Smarty S67