The recommendation didn't have anything todo with the DEF being different, they are the same... Or the dealer doesn't know what they are talking about. Either very possible.
The baby DMax requires a specific DEF, its API certified or ISO. 22241 which is basically every DEF in a jug and all of them being sold in bulk. DEF does go old, it doesn't like heat and it freezes when it's cold. The benefit of bulk DEF is that they go through a pile of it, that material is turned over quick vs a jug which could be old (they have date codes, you can check them before you purchase).
To fill or not to fill the tank. There isn't a reason to have a full tank of DEF beyond convenience, nothing bad occurs to a partially full DEF tank (it's already mostly water, full doesn't help you). I put 15k miles on per year on both the baby diesels, I dump in 5 gallons when they get low; so far so good.
