I use Royal Purple and have it changed every 10K, and the fuel filter about every 7.5K unfortunately. I can always tell when it needs changing because my mpg really drops. I have recently had the TV "decluttered", chipped and tuned (5 different programs), so I am anxious to see how this mileage will differ when the 7.5K comes back around again. Mine gets driven all around, so I really notice when it heads south.
2012 Dodge 3500 Cummins Crew Cab 4X4, "Decluttered" and Tuned/2013 Forrest River Viper XLR 390V10 Toy Hauler/2012 Harley Davidson Ultra Glide CVO.
Full time RVer.