That overlaps with fuel path finding code for routes with intermediate refueling. Currently, AI does not use such routes but there is HRM#927522, and surely players would like to get true numbers of turns to get somewhere.
alienvalkyrie Could we call it "Fuel_Bonus" instead? To be in line with "Move_Bonus", and to avoid confusion with (terrain) extras.
Effect to give units extra turns of fuel, checked at refuel time, could be useful.