You'll be fine with either. Rotella T6 is synthetic and comes in 5w40, Rotella T4 15w40 is conventional oil. I'm pretty sure you won't find a full synthetic in 15w40. Factory fill from Cummins is Valvoline premium Blue 5w40 full synthetic because they don't know what climate they will be operating in. It's fine to switch between the 2 weights. I use VPB 15w40 or Rotella T4 15w40 depending whom has it on sale at time of oil change. Currently using VPB, I also keep 3 gallons of oil and a extra filter between oil changes so I don't have to hunt for sales. When I use the spare oil I go on the look out for sales to restock. Hray