DEF is around 1/3 Urea (not the same as peeing in it lol) and 2/3 water (de-ionized). I used to work at NAPA, and you can find all sorts of brands. The fresheness is not really an issue, however the price can be. You can usually find DEF on sale, and in 2.5 gallon containers with spouts for a clean fill up. I tend to go with Blue DEF made by Peak just because it is prevalent in my area.