I have a 2007 Dodge 2500 with the 5.9. Started using Amsoil at 15K miles. Now at 145K. Amsoil is PIA to get and it seems from reading the Rotella-T may be a good alternate. any suggestions. Been running 15/40 year round.

Been using regular Rotella 15-40 in my truck since new. No complaints.... just don't be suprised if you catch an "odor" for the first 200 miles or so after an oil change. Nothing offensive, just a "rosmella" odor.

That 15W40 is pretty thick to run in a newer motor, especially in WA. What does the manual spec? How long are your OCIs? That will determine if you need synthetic or not.

It's a Cummins. They spec 15-40. Only recommend synthetic at temps below -13. Rotella T is just fine. If the OP still wants synthetic he can go the Rotella T5 sythetic blend, or T6 full synthetic.

Who carries T-6?

Walmart usually has T6 full synthetic on hand.