Hmm, that is weird, when I was working for mopar, our shop opened those cummins, never heard of us having to outsource our engines to the manufacturer of the engine. Maybe that has changed since I stopped working for mopar. I stopped working for them back in early 2000's. Wow time flies.
I see the OP lives in arizona, so anything 15w40 is fine. Valvoline makes a good oil for these cummins as you wrote in your initial post, also amsoil, penzoil, shell etc.
My take on this very old debate is that if the oil is rated for a certain application and is regularly serviced with quality filters, no dealer can say shyte regardless of which BRAND is used. This is what you want to see on the label of the bottle, what ever the brand. If that is there, your golden.
If your going to go good oil and do it yourself, try this from your local walmart. Hard to beat 3 gallon jugs for 40$. just what you need for service. Spec for oil change is 11.4l.
This a great oil for your truck at a good price. Add to this a fleet oil filter, what can go wrong at that price? Sure it's a synthetic blend, but if you go rotella, thats 22$ a jug, so that is 66$. So that's 22$ you can invest in for a good oil filter. Hey, If I can save a few bucks here and there, why would I buy glorified diesel oil. This is not a ferrari. Just a good old cummins.
The main concern here is regular intervals. As these cummins do soot up quick with the turbo, EGR and all. So even if you have full synthetic in there, it will soot up as quick as regular or semi syn.