My mother in law lent me her car for the month I just filled up the entire tank before returning it.
Is that somewhat assur because of ribbis?
That was the correct thing to do, because that is simple hakaras hatov fopr the fact that she lent you her car for free. There is no issue of ribbis here because ribbis only applies to lending money or something that you borrow one thing and you use it up and return something else in its place, such as money, eggs etc. When you borrow something and return the actual item itself, it isn’t ribbis and you may even pay the person money to use it, such as when we rent a car and pay the owner money as rent. It isn’t ribbis, and permitted.
Baba Metzia 69b, Y:D 176-2.