DEF - Diesel Exhaust Fluid
Def is nothing special as far as one brand being superior to another. The cheap Walmart brand works just as good as the higher cost stuff. As Ray said it’s all about the age and what temperature it was stored at. So I buy my def based of what goes off the shelf fastest, and here they have a hard time keeping the Walmart stuff in stock so that is what I use. With my 2019 F350 6.7 diesel once the def gauge is at about 1/4 I put in 2 of the 2.5 gallon jugs and it fills it back up and I am good for about 4,000 to 5,000 miles of driving. I have heard stories of dealers accidentally putting def in the diesel tank so I will never have to hem top it off for me. If you ever accidental put def in the fuel tank do not start the truck and have it towed to the closest dealer. If you start it you are looking at a complete fuel system like Rush mentioned.