I friend of the family drove a older 12 valve 5.9 in a dulley 3500. Never did short city driving, all long haul. He was running a GSA contract run from Seattle to LA, then to NY, NY and back to Seattle pulling a 5th wheel 6 car ramp trailer. Last I knew he had over 1.2 Million miles on the truck and never has had any engine issues. He was doing the 3,000 Oil Change Interval until warranty run out.
He then started doing Blackstone Labs testing. He did add the AMSOIL by-pass fitration system and Blackstone tests had him just changing all oil filters at 15,000 miles and with the filter makeup oil and what oil added as needed he was at about 50,000 miles before Blackstone said to dump it. They said the oil was good for another 15,000 miles but it would then be borderline on the TBN scale. Since he was doing about 100,000 miles a year it was two changes a year, Rotella T6 5W40 winter time and T5 15W40 summer.