At the very least, a diesel should be running a 15w40, unless its cold (0F and under). Rotella T is, IMO, a good non-synthetic oil. I think its better than many synthetics out there, like Co-op synthetic.
I personally like 5w40 synthetic oil for year round use. Again, IMO, PetroCanada Duron 5w40 Synthetic is about the best oil you can buy. Its got the highest flash point of any w40 oil going, so it responds well to high turbocharger and oil galley temperatures without breaking down or coking up. With US guys, its tough to find PetroCan oils, but there are some PetroCan dealers down south.
I know many people who have run Amsoil synthetics and had great luck with them as well. I don't much care for the fact that Amsoil has never taken the time to get an API rating. They say they don't need it, but I've always wondered if all they're doing is hiding something.
Maybe I'm just a suspicious sort
Rod