My dad ran 10w30 T5 in his 2012 6.7 for the first few changes, but has been using Mobil Delvac 15w40 since. He's convinced that the 40wt is smoother and quieter, with no measurable difference in fuel economy. They are all approved in the manual, and the Delvac is cheaper while still being a great oil. If it were mine, especially in Texas, I'd just run whichever 15w40 HDEO you prefer.