I have 2012 Dodge 3500 Cummins Dually, the last year Dodge diesels w/o DEF. 12.5 -14 mpg driving around town and highway combined. I change my oil every 4-4.5k miles, it gets black with carbon, but I only put on about 8k miles per year on the truck, so its not too bad. Def trucks, from I read on truck forums go longer. Emission sensors on diesels are a PITA, more CEL over gas trucks because of the EGR pushing carbon into the turbo. EPA has all todays diesel trucks smogged down horribly. Getting a pre DEF with low mileage <80k may be a tough search . I wouldn't go back to a gas rig for what we do with our truck, been there with a gasser. Expect 9.5-10.5 mpg average with a cabover, its a lot of air to push out of the way. Towing my 7k fishing boat that has a Pilothouse that is 10 1/2' high on the trailer, boat only, about 10.5-11 mpg to the harbor and back, half freeway half city traffic, interesting comparison between towing high profile and having it on the truck,,,gregg