I regularly see 20 to 21 mpg on the freeway on roadtrips in the summer monthes. The avg is around 18mpg with mixed city and highway driving. The best I ever saw out of my V-10 reg cab 4x4 dually with an auto and 4:30's was 12mpg, normaly it was closer to 10 or 11mpg.
Whether or not somebody beleives the mpg numbers I have posted , or thinks I bought the wrong truck, the wrong engine, or the wrong color is of no concern to me , as I bought it to serve me, not them.
I agree a gas engine in a fullsize pickup today can see 150k or more if it leads the proper type of life. I bet a v10 ford, 6.0 GM or a 5.7 hemi dodge could obtain that pretty easily if it were used as a commuter most of the time, and did not tow any real weight on long grades. However, hitch up a 28 foot enclosed with a race car in it, and tow up through Fancy gap in Virginia 5 or 6 times a year with one of those gassers screaming at 3500 or 4000 rpm for 4 or 5 miles climbing a grade, the odds are you will be changing that gasser out at around 120k or so. I am basing this on a buddies V10 I replaced for him about a year ago with a broken timing chain, and my brother in laws old 1500 ram with a magnum 360. Two cracked cylinder heads in 5 years before he traded up to a diesel.
There is considerably more to picking the correct engine for the application than a couple of mathematical formulas for oil changes and fuel cost on a message board. How about how much weight will you be towing for how many miles through what terrain at what elevation, do you own a business with your own fueling terminal, if so, what fuel are you set up to dispense, will the vehicle be used for short commuting or long trips, winter driving, if so how cold etc. etc.
It always amuses me when somebody thinks that they bought the only good type of truck, and every one else with something different bought one that was to under powered, or spent to much on a big engine that they probably almost never need anyway. The reality of the situation is the reason we have so many different combinations of chassis and driveline configurations is because there are so many people with so many different needs to fill with they're trucks. One mans perfect do everything truck may be another mans lemon. Buy what you like and be happy with it, don't waste your breath trying to convince somebody else they should have bought exactly what you did.