when researching the refueling system (the one that allows you to transfer more fuel per hour) once you research the one before 125,000 liters per hour it shows all of the remaining techs for that branch
e.g.
refueling system: 125,000 Liters
refueling system: 160,000 liters
refueling system: 200,000 liters
refueling system: 250,000 liters
refueling system: 320,000 liters
refueling system: 400,000 liters
refueling system: 500,000 liters