Sounds pretty long interval, but if you analyze the oil then it must be ok, do you usually drive only highway runs, what makes the oil last longer? I haven't used that long intervals in any of my cars, but then again i haven't ever analysized the condition of oil...
You could go 12-15k miles on dino oil with a bypass filter, which is what the big trucks have been doing for decades, otherwise, 25 years ago diesel oil was only rated for about 1500 miles between changes without a bypass filter by the API ( american petrolium institute ) but with the Lubrifiner bypass filters, 12-15k was fine, then they went to spin on bypass filters on most big trucks and they are less sooty than before, I believe they are going even longer now on dino oil. With synthetic, the only limit with a bypass filter is basically if the oil becomes contaminated with coolant or fuel dilution, or some odd mechanical oddity from an older engine. It's quite common for oil to go over200- 300k on a sound engine with analysis .