I have ran it lower then this before (according to guage) and didnt have any issues. I just thought it was awful wierd how it was running fine when I pulled in and then a minute after I bought a little fuel it died. I will be sure to keep it above 3/8 of a tank from now on.
A lot of buses don't even have fuel gauges. Most people just fill up conservatively based on miles driven. Older Greyhound buses never had fuel gauges. Greyhound knew how long routes were and where buses needed to be fueled at.