one key problem behind current "fuel" system is very low, non-upgradable fuel reserve. With regen being fast, this basically causes players to visit the game many times per day to use energy, then quit until energy is restored (don't tell "they only will do that if they want", desire to get something non-free wlll drive 95% of players who still need some items to do that). if regen would be slow, this would cause another problem of too slow campaing progression and money grinding. Unability to do 10 missions in a row is a terrible thing.
suggestion: as level ncreases, increase max fuel (and probably fuel regen, too).