The metaphorical container that holds all the liberties in a friendship. Occasionally needs refilling to keep the friendship in a proper give-take equilibrium. Can be refilled by one friend giving the other friend food, alcohol, sexual favors, money, rides, or even filling their actual gas tank.
Molly: "Mike, can you drive me around to do some errands today?"
Mike: "It's been a while since you filled up the friendship gas tank..."