I've gotten 17.1 gallons into mine by testing just how far I could go with "0" DTE reading on the OTIS. Needless to say, I won't push it that far anymore, but yeah, I was on zero for a long time. Either 17 is a rounded off number or I was riding on fumes.
And yes, the OTIS is being very conservative. From my experience, there is about 2 gallons left in the tank when mine reads zero DTE. Also, on the highway, the DTE doesn't start counting down until 30 or so miles of driving so it doesn't really count that top 1 gallon either.