I've also been questioning this. Manual says 8000, dealer says 3000 but my GC has the oil change indictor to inform me to change it, seems confusing. Here's a good link to some maintenance schedules,
Jeep Grand Cherokee WK2 - Maintenance information and schedules
The owner's manual gives the Maximum intervals and the dealer seems to give the minimum seeing as they want you back to see them more often. I will be changing the oil when my jeep tells me too.
So far on my third oil change my Jeep has asked for changes at 3500, 7500 and 10750.
When it comes down to it go with your personal preference, just make sure to keep an eye on your levels.