On new(er) diesel trucks, they require 2 things Ulta Low Sulpher Diesel, and DEF. That's Diesel Exhaust Fluid. And, yeah, it's real. It's also fuckin' expensive! A gallon of diesel is $4.15, give or take $0.20. The average gallon of DEF is 12 - 15 dollars. So, why is it so special? I was curious, so I looked up what it is. It's 34% Urea, 2% trace elements, and 64% water. Now, for those in the know -- that's PISS. Yeah, less minerals, and bacteria, but, it's 12 dollar a gallon PISS (unless you are refilling your DEF jugs -- nor recommended, it'll void your warranty.)