I bought a 2012 Cascadia with the DD15 that had been on the lot for about a year. We have had a handfull of emissions issues that the mechanics say are probably due to the truck sitting so long, but in 30,000 miles I haven't added a drop of oil, other than an oil change (25,000 miles ago). The oil sample I did last week came back clean, with only the base being flagged at a level 1 with no bypass filtration.
We are getting about 5.75 MPG right now, but have a lot of idle time. because the APU also has problems, so wont stay on over 2-3 hrs at a time. This combined with the emissions issues that are forcing us to regen almost daily (after extended idle periods), are bringing the numbers down.
We are headed to Pittsburgh Power in a couple of weeks to get the emissions systems, crank dampener, oil bypass filter, Dorothy, and the engine flush done. I have high hopes that even with the idle time (APU is my next repair goal). our MPG will come up at least one mile.
According to Fuel gauges reports, this combination should be good for around 7(ish) MPG.
Ultimately I think that this is a good combination, just make sure the emissions systems have been serviced recently, so you don’t have those issues .