Fuel is not included in the price of a rental. You should therefore return the vehicle with the same fuel level as it had at the start of the rental.
If the fuel level is lower at check-out, the owner can adjust the price to compensate for the difference.
If the fuel level is higher at check-out, you can make an adjustment yourself, if the owner didn't take care of it for you.
Note: For Getaround Connect rentals with automatic fuel adjustments, you will be charged an additional 0,60€ per missing liter (0,60£ per missing liter in the UK) for the refill service.