It would be wrong anyway: Vol in ounces = Vol in gallons * 128
It looks like the software only uses the units to report. This means the volume is always in base units and is adjusted to display the converted number rather than scaling the value and using that.
Either it's a bug or a lousy way to meet a badly written software specification.
You can create your own units.
Does the ounces parameter have units?
It's possible the software is ignorant enough that if it was also in gallons the calculation would work.
You know the units would be wrong, and I know the units would be wrong, but I'll bet a note on the drawing wouldn't care.