I`ve seen many cars/trucks that I had to repair/clean sludge out of and/or replace the motor due to being sludged up from following the manuf. recommended interval. enough that I`m very happy I have always done all my oil changes at 3000-3500miles (4000-4500miles on synthetic!) in my gas motors. anything more and your asking for trouble. I`ve seen first hand what can happen running the extended oil change interval! the manuf. only really want the vehicle to get out of the warranty period, after that any repair is on your dime.
My V-10 X has 83k miles on it and it has had the oil changed every 4k miles (Mobil 1 synthetic), trans fluid every 25k, coolant at 50k, and rear diff every 25-30k miles. fluids are far cheaper than an engine, trans or diff repair!
02 Ford Excursion V-10 4.30 gears 5 Star Tuner Y-pipe mod Hellwig sway bar Rear Airbags
Prodigy and Dual Cam
07 Cherokee 32B
Better to have a bad day of camping than a good day at work!