The CEL is completely independent of the Change Oil message. You should have your dealer check the CEL as soon as you can. You are most likely going to see a p2000 or p2262. The p2000 means you have an o2 sensor that needs cleaning and/or replacement and the p2262 means your turbo needs cleaning. These are the two most common CELs that have been seen from these 6.7L equipped trucks.
Agreed, with one possible exception: If you have something going on (that caused the CEL) that makes the truck regen more frequently it could make the oil change message appear sooner.
The oil change message does NOT sense the viscosity of the oil. It uses an algorithm based on engine hours, miles driven, idle time, regens performed, etc. Thats why the message can seem to come on at such widely different times from truck to truck.
Philosophy on changing oil is a near violent topic on this forum so I will tell you this about my experience and you make your own decision:
1-Other than the free oil change from the dealer at 5k miles, I have never changed my oil sooner than 7,500 miles.
2-My most recent oil change was 9,400 miles and it was on a round trip to Alaska including up the dirt haul road to Prudhoe Bay.
3-I always have my oil tested by Blackstone labs. The 9,400 mile sample was fine and the oil still had plenty of life left in it with minimal sediments and good viscosity.
4-Yes, when I change my oil and I refill it the dip stick shows it right in the middle of normal. After 3K - 4K miles it always shows 2-3 quarts higher. However, the oil sample testing always shows the fuel in oil level at <.05% so it's still perfectly safe. I just don't worry about it anymore.
IMHO if you want to change your oil every time the message comes on, go ahead. Its a cheap insurance policy and you cant hurt your truck that way.
If you dont want to waste money on excessive oil changes you can just decide to change it every 7,500 miles or minimum once a year, which ever comes first. Unless you have a reayy tough duty cycle for your truck you should be perfectly safe.
You may want to consider getting an oil analysis done to establish a baseline for your engine then get it tested every 2nd or 3rd oil change. You'll be able to track how the engine is wearing and it can gives you a heads up to a potential problem if something changes, like increasing fuel or anti-freeze in the oil compared to previous samples.