No, you're not missing anything, the ISX has different change intervals. If you're worried about the time then pull a sample and go by it.
My ISX hasn't needed an oil change since May 2015, only about 4,000 miles in that time but the samples come back saying to run another 5,000 miles and check again.
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2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft, HWH Active Air
Charter Good Sam Lifetime Member, FMCA,
RV'ing since 1957, NRA Benefactor Life, towing '21 Jeep JLU Rubicon Ecodiesel
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