DEF should be stored in a cool, dry, well-ventilated area, out of direct sunlight. While the
optimum storage temperature is up to 77 deg F (25 deg C), temporary exposure to higher
temperatures has little to no impact on the quality of DEF.
Of course any DEF that's in your reserve tank of your vehicle where it's supposed to be is
fine.
Do not run low on DEF and top it off every 2,000 mile if you want. I don't have a DEF
gauge, although, Mercedes says it should last between services provided you don't have
a malfunctioning system using excessive DEF.
I used to carry spare DEF, but, it's just a waste of space, you can get it from any truck
stop on the road. My advice would be to carry one 1/2 gallon bottle, Mercedes sells a
very convenient one that you just unscrew and twist. It's expensive for what it is, but, it's
VERY easy to use.