JMHO
While the quality of the oil has continued to increase (New CI-4), the demand on that oil as far as our Cummins engines go has probably became easier. My 12V while very clean running can not compete with the clean burning that the new trucks (expecially the 2003+). They never smoke, NO ERG, very clean burning, they run warmer (then our colder running 12Vs). All this means much easier on the oil. I would venture to guess anytime a manufacture increases the oil change interval it is backed up by data breaking every rule but the mileage interval and yet no failures. The 2004. 5 engines are probably the cleanest buring (easiest on the oil) diesel engine made.
Here is my experience with oil change interval in a older 12V that is much harder on oil then todays trucks. I operate my truck breaking almost every item listed in the above list that should mean use Schedule B. I started out with 6K changes (this was the Scheduel A back then... B was 3K) and got tired of dumping 3 gallons every three weeks. I had 60K of oil samples showing the oil perfectly fine for contiened use at 6K. This is with lots fo very heavy towing, in town short trips, and arctic cold winter operations were oil temps would not go above 170 for weeks on end.
I swithced to snythetic oil but no bypass filters system. I found that even out to 24K miles, the oil is just fine. Even while continueing to tow heavy, short trips, -30 winter freeze your azz off operation. While the synthetic oil helps I personally believe it is the clean burning design of these engine that keeps oil in check longer.
Those of you that change at 5 - 7K, I bet the oil has not even started to get very dark yet.
If I bought a new truck, I would without question run at least 10K miles between changes. I would start pulling samples after 10K on the engine at each oil change and use that to push it out to 15K. At the very least changing the oil filter 50% through your dump interval.
What effects oil samples (in my case), Towing heavy in summer heat will cause wear to go up some ( was OK even with 5W30 but think 5W40 to be alittle better). Making lots of smoke (turn up the power) will cause soot to go up some but agian nothing that cause one to shorten the schedule. I never idle but can see that this would not be good especailly if it was not followed with a good heavy tow to heat things up. I was suprised to see that very cold winter operation did not cause much change. Soot never ever got within 20% of the flag point and yet the oil was blacker then black. Never any fuel dulution (winter or idleing).
If you have oil samples from your 2003+ that show oil problems with drains beyond 7500 that would be go to know. I don't think even with heaving towing, winter use, short trips oil drains with regular oil and filters will have a problems out beyond 12K or more.
Sorry to ramble... .
jjw
ND
Attention: TDR Forum Junkies 
you need to use schedule B which requires oil changes at 7550 miles. 
That was unheard of then, and way beyond the 5000-7500 Chevy was recommending at the time. I'm kind of with Gonzo 1066-I may go 15000 when it's not below 32 and when I'm not pulling...
