Like someone mentioned I would let the DEF tank get low so you can reduce the average age of what is in the def tank. We have had a few DEF pickup trucks here at my work and the DEF systems have given very little trouble. Each truck has had a DEF tank and heater replaced once in its life but beyond that no other problems. When we quit driving our ford truck every day it eventually came up with a contaminated DEF code that would come back as soon as you cleared it. It had a 1/2 tank of DEF so I topped it off with fresh and the problem hasn't come back. The 2016 chevy I use as my personal truck uses so little DEF with my driving that I worry about it getting old so when it says it is low(no gauge) I only put 2.5 gallons in it instead of the 5 it holds.