When buying the jugs, in addition to checking date codes and safety seals, also be sure that the fluid has been stored somewhere out of direct sunlight and preferably in a climate controlled portion of the store. A lot of these gas stations and auto parts stores love to store the DEF in the entry way, where they’re bombarded with sunlight, and experience temperature fluctuations repeatedly.
Buy the ones most recently manufactured if possible.
Best practice for the DEF system in these trucks is to run the tank down to 1/4 level, add one jug (2.5 gal) and that takes you to 3/4. Run it down to 1/4 and repeat.
Repeatedly and consistently running the DEF level down below 1/4 or 1/8 can significantly increase the risk of crystallization contamination on the DEF sending unit. Likewise, filling it more than the max capacity on the tank can also create issues. (Specifically where the level sensor can no longer detect the fluid level). There’s a gap of air that’s required at the top of the fluid in the tank. When that gap gets filled with fluid, you get a “no-echo” situation from the sonar-style level sensor.