IMHO, synthetic is the only way to go, and my preference is Amsoil. In fact, I will be changing my oil tommorow... Using the tried and true Amsoil HD Diesel and Marine 15w40. I just had oil analysis done on this last batch and it said it was still fine to run. That was 5k miles ago. This batch of oil now has 20k miles on it and I am betting it will still come back as ok to use, but I will be changing it anyway.
Scott