do you fill your washer fluid when it's empty? Urea is NO different, the cost is minimal and the fuel mileage gains make it nearly negligible, there are less regens, no cylinder fuel washdown(like on the dodge) with the 9th injector in the exhaust, and when comparing power there is almost no difference between them, and from what I understand the Allison and the turbine breaking work better than on the dodge, has better ride and handling as well

 

also there is supposed to be a noticeable differences in mpg between the Dodge Cummins' nox trap and the ford/gm urea injection. you will probably come out even or ahead financially with the urea system

 

I just purchased a 2012 GMC 2500 with the LML and love the damn thing, 1300 miles no urea yet but getting close to 20 mpg on a 300 mile round trip, has more power than needed.