I gotta go with the theory of following the manufacturer recommendations with the caveat that you only use, and stick to, the severe service maintenance schedule. For my wonderful old 2nd generation 5.9l diesel RAM that means every 6,000kms (3,600mi. I run a Rotella T6 synthetic 5W-40 oil. It also tells me to change the fuel filter every other oil change and I know that some members on the diesel forums go 50 or 60 thousand miles on a fuel filter. Regardless of the myriad of opinions out there, of which there is NO END, I feel that regular and frequent oil and filter changes is cheap insurance and peace of mind. As always, YMMV.
Cheers!
Mike