I totally agree with geof.....my FL freightliner had 300,000 miles with a 3126 250hp cat.....started to have a little blowby...only put in one injector and one wheel seal in 300,000 miles....run it everyday pulling 12,000 lb trailer.....trans was tight and so was rear differential. Cat master shop told me that this engine operating life was 30,000 gals of fuel....10 mpg....equals 300,000 miles. Some will go 500,000....but they are expensive to overhaul!!!!! They are great trucks some of the newer business class with newer engines may go longer, as am not sure, but they get better fuel mileage.
Good luck to you!
Holy Cow!.... a loving younger wife that keeps me going, a 6 yo little lady that is a princess/tom girl, a 14 yo that thinks he can sled better than a 30 year veteran sledder, and a 19 year old that is a soldier...