This can be somewhat of a can of worms... are you running dino or synthetic oil. . what filter are you running a second filter... you could just take regular oil samples for testing and wait for the results to tell you to change the oil... ... .....
IMO, if your running the Fleetguard Filter and dino, blended or synthetic oil I would give you 7500-10000 miles, if your putting any sort of towing or heavy loads I do it 3500-5000. You can find out a lot of information by searching for "extended drain intervals" and the like...
Exhaust Brake... are you going to become hopelessly addicted to your truck and the fine art of bombing?... . For mild applications I think the Jake, BD, Pacbrake are all fine. I have a Jake many members swear by BD and Pacbrake..... If your going to start bombing the truck get the gauges and a 4" exhaust from the turbo back and put a BD 4" exhaust brake in at the same time...
my 2 cents
-Matt