GF,
don't have the specifics on the product .. but, if you measure quantity added in volume (litres, gallons, whatever - as is obviously the case from your post), one has to do the conversion from required mass at the relevant SG to come up with the number of volume units for the bowser chap ... pain in the neck but only a minor nuisance apart from the philosophical problems associated with risk of error.
.. then again, you might talk one of the commercial operators into requiring a change in the system (at huge expense) so that the gauges all read mass units ... ?
trust that you have recovered from the rigours of the Avalon airshow ...