An interesting bit that I have learned is that the new Shell Rotella T6 in 10w-30 has a kinematic viscosity at 100°C of 14.7 cSt; that's a 40 weight viscosity, not a 30 weight. For comparison's sake, the Shell Rotella T6 5w-40 has a 14.9 cSt at the same 100°C.
Shell confirmed these numbers were indeed correct and also said they are the factory fill for the Cummins. Interesting, since Cummins otherwise has an endorsement with Valvoline much like Mobil1 does with Corvette. The Valvoline Premium Blue One Solution Gen2 10w-30 has a kinematic viscosity at 100°C of 12.0, indicative of a true 30 weight oil. So the question is, what does Cummins really feel is the better oil for your 6.7? If they are conceding a 10w-30 because of the climate change terrorists at the EPA, did they have a hand in the new Rotella T6 10w-30 looking like a 40 weight oil in a 30 weight label?
If you want to run the oil that came in your motor from Cummins, and is what the dealer supposedly uses, that would be Shell Rotella (assuming it was T6 but have not been able to confirm as of yet.).
There is no need to use the 10w-30 variant, just get the 5w-40 which can be had easier and for less money.