Aries,
Although I agree with you on this, I don’t understand your distinction between a favor and a mitzvah. It’s a mitzvah to lend your friend a car if she needs it, and although not obligatory, it would enhance the mitzvah if the car was clean and fueled. Yet, if you did’nt clean or fuel it, it would be obnoxious of the friend to complain (unlike the case of the late ride, where it was an “avla” on the part of the girl to show up 1 1/2 hours late).