I was told that if you borrow a car from a friend, it's courteous to return it washed, and with a full tank of gas (even if the car was dirty and had an empty tank when you borrowed it). Is there any problem here of ribis/usury/interest on a loan?
Ribis is not on an Item that you are going to to return the very same item itself, so essential Ribis is only on Loans of money or things of Monetary value, not of items where you return the actual item itself.
To illustrate for example borrowing eggs(this is Monetary Value) and returning more eggs since there you are lending the value of the item not the actual eggs it is not allowed again monetary value=Ribis. That is even if the price of sugar goes up from when you borrowed it the problem of Ribis applies. so you are allowed to borrow the car and fill the tank.
For a more in Depth treatment see Here: