Thanks for this, as I mentioned I am new to the diesel world and have plenty to learn just like I did with any of my other vehicles.Isn't there two schedules for changing the oil, one for what they call severe service which includes short commutes, towing and dirty dusty conditions among a few other things and then the normal schedule where you have longer trips of 20 miles or more and do not encounter dirty dusty conditions and do not idle the engine for long periods of time. I would follow the manual if you meet the severe conditions then change the oil at that point the manual calls for, if you meet normal conditions then run the full 15,000 miles. If the oil change is the same for severe and normal than follow the manual. Cummins does not want their engines blowing up so there 15,000 miles oil change interval is sound.
Oil today can handle well over 15,000 miles, if you really want to know the condition of your oil send a used oil sample to a company like Blackstone or one of the others. Just remember that they are measuring in parts per million so don't get spooked by readings, every engine wears differently one engine might show more iron wear than another, or one might show more aluminum wear than another.
I run Redline oil because it is a Group V Ester Synthetic 15w-40 oil, were you live you will need to run a 5w-40, if you wanted to run Redline in that weight you would have to use their professional series oil which is a Group III/Group IV PAO blend 5w-40.
I live in Southwest Florida where our winters are mild and a 15w-40 oil works fine for the temperatures we see. If I lived up north I would look for a 5w-40 oil that is a Group IV PAO with Group V Ester blend synthetic or if you can find one a Group V Ester synthetic.
Apparently Normal operating and Severe operating condition call for the same mileage schedules for the 2019 plus 6.7 Cummins, for an oil change it is 15,000 miles for either one. Transmission and differential fluid changes are different schedules for 2019 plus.
Here is a link to that schedule for 2019 plus: 5th Generation 6.7L Cummins Maintenance Guide And Service Schedule
With regards to oil change intervals and best oils to use, my inexperience in diesels or any other engine for that matter, tells me to error on the side of caution and more frequent is best. In this case, the fresh oil (I’ve only done one oil change on the truck) is black almost as soon as I put it in the engine so does it not make sense to change it more often given that the truck sits most of the winter? Come the spring it will be used more for what it was built for, in our case pulling a 43 foot RV, and perhaps oil change intervals can be extend.