Adblue or Diesel Exhaust Fluid

Adblue is just one brand name for DEF. The arch-nemesis of DEF is heat. DEF degrades over time, but its shelf life can be greatly extended if stored sealed in cool dark places. Some reports indicated a well stored DEF can still be concentrated enough at the two year mark. It's all about concentration, not age. If topping off has raised the concentration enough, then you're good. I never top-off. I let the DEF go almost empty, then fill about 2/3. My JGC is also parked regularly in a closed garage in the northeast so DEF degrades relatively slowly. If you end up with low concentration DEF in your tank, the ECM will notice DEF low efficiency and issue you an alert, and eventually limit your mileage. In such cases (and if you're low on DEF), topping-off might help. In other cases you'll end up requiring flushing the DEF tank. If you have extra unopened containers you're not sure about, you can use a DEF refractometer to measure and verify DEF concentration.

I generally keep the tank less than half full unless I'm planning a very long road trip. I usually buy 2 of the 2.5 gallon boxes in the late winter when I can assume they haven't sat in a hot warehouse for months since the cheapest brand is made about 300 miles from where I live. When COVID hit, the DEF I bought in 2020 sat in our basement for about 2 years because we didn't drive much and it was still OK since our basement stays nice and cool all summer.

I generally buy DEF from the pump at a truck stop - cheaper and always fresh. No need to store it or try to fill it from those stupid bottles it comes in.