I stopped buying the Peak brand, even at Walmart. Figuring the $/gallon price, it is cheaper just to buy it at the truck stop at the same time I am re-fueling. My Mercedes Sprinter has a builtin gauge in the instrument panel that allows me to see the DEF level (plus there is a dash light that comes on warning the DEF is getting low). I know that I need to top off the DEF when I've gone around 6,000 miles. I usually stop at Pilot/Flying J or Love's for fuel. The DEF is way cheaper, there, and can count on it being fresh, plus i don't have to find a place to store the box in one of the basement compartments, if I don't use the whole box.