[davemitchell59]

This all seems contrary to other threads and, I think, the Duramax manual. Seems like most recommendations are to run it down below 20% before adding DEF. I recall reading somewhere that you could mess things up if you added DEF before you got to the 20% warning.

My 2016 only reads "Ok" until you get to the 1000 mile or 20% level. Are we suppose to guess at the level and add 2.5 gal every 3000 miles regardless of indicators or recommendations to let it run low before adding?