No big deal, have only "messed" with it for 2,000 miles, added two gallons because it wasn't full when I got the truck on May 9th. Have a gage, I'll use it, plus you have warnings if and when you need them. I purchased 2.5 gallon Blue DEF for $11, "just in case"...carry it in truck tool box...Only time I added was at truck stop for $2.89, and again, two gallons filled it to the top.....don't know how much was in there to begin with, but it wasn't full.
Cheapest is to find a place that has it at filling station and add DEF there...Have a place about 5 miles down the road, diesel pump and right beside of it is a DEF pump...
As stated above, better mpg with DEF equipped vehicle..I owned a 10' and 12' Ram dually, same configuration as 13' in my sig. and I'm getting 2 mpg more with the DEF equipped 13' than the other two...