The 15w-40 is the standard fare in the GM 6.6L V8 Duramax diesel and an option in the Ford 6.7L Powerstroke, but with the hydraulic lifters in 2019+ Cummins specifically says not to run the 15w-40 oil.
It gets below 0 degrees for just 3 months? Remember, the Cummins via the owners manual is talking about 0°F here, not °C which is 32°F. It might get cold enough to snow or even get below freezing for a few days at a time here in Texas, but it doesn't get below 0°F.
I haven't decided yet which oil viscosity to use when I get to the my first oil change on my new MY23 Ram 2500 Cummins. Coming from GM I believe their 0w-20 viscosities were heavily influenced by the EPA and hated to use it on my 6.2L V8 when 5w-30 and then 0w-40 are what they are calling for on the same long block in other vehicles like Camaro and Corvette.
An interesting thing that I wanted to share and get other's here's opinions is on the Shell Rotella T6 10w-30. Although recently released, it's still hard to get but you can order it on Amazon for reasonable prices. But it's product specs are what prompted me to clarify with Shell whether or not they were indeed correct. Shell is saying that their Rotella T6 10w-30 has a 14.7 cSt at 100°C kinematic viscosity which places it in the 40 weight oil category, not the 30 weight that the rating on the bottle says. Most other CK-4 10w-30's are running between 11.0 and 12.1cSt at 100°C.
The Shell Rotella T6 5w-40 only has a 14.9 cSt kinematic viscosity at 100°C for comparison's sake. Given that, the oil viscosities appear to be nearly identical, but I do like the 5w-40's -51°C pour point a lot better than the 10w-30's -39°C, not to mention that the Shell Rotella T6 5w-40 is widely available and even less expensive.
Another interesting tidbit is that while the owner's manual specifically mentions the Shell Rotella by name, Cummins officially endorses Valvoline, (kind of like the way Mobil1 is endorsed by Corvette.) So if Ram and Cummins are using Rotella T6 10w-30 as factory fill, they're essentially using a 40 weight oil by spec and that is in part what makes me so unsure as to which direction to go. If a high 14 cSt is indeed what Cummins prefers, I'd rather go with the 5w-40 Rotella T6 with it's -51°C pour point.