FILLING UP THE GAS TANK
Sometimes a person lending their car to a friend may request that the car be returned with the same amount of gas as when it was lent.
Is this an issue of ribbis (seah beseah)?
Contemporary poskim permit refilling the tank before returning a borrowed car, since it is obviously being done out of gratitude and
not as an additional payment for a loan. The same applies even if the borrower returned a full tank. When the lender requires that the gas be refilled, certain poskim require that one specify that the refill was in gratitude for use of the car. See also “Gifts of Gratitude.”
Note: Renting a car from a Jewish company which requires the car to be returned with a full tank of gas, may be a problem of ribbis. The company should ask a shailah as to the best way to structure their rental agreement in order to avoid this issue.