There is NO way to programmatically control fuel levels in a multiplayer race using InSim because you can't access the fuel levels in multiplayer. However, if you're going to be using singleplayer only then it is possible to use OutGauge to determine the fuel level in the car. By analysing an SPR (singleplayer replay) you could tell what the fuel level in the car was as soon as the replay started or when the player left the pits. If you want to do that I can provide a small example in Java to get you started with.
To the guys above...what was the point in posting if you didn't know what the OP wanted? He asked in good English, so it wasn't a question of understanding broken English written by someone using a translator. If you don't know anything about something don't post. Go back to spamming Off Topic