I need a little advice on DEF, I have 2.2k miles on mine and I need to start thinking about which DEF to use. Which brand do you guys use? A buddy of mine also has a 3500, and he said I better us Ultrapure or I'm asking for problems. He said his system needed to be replaced because he didn't use Ultrapure from the get go. Is there any truth to sticking to certain brands?