The Key'n Go fuel policy you select in Benfica, Lisbon will determine your fueling responsibilities. With a full-to-full or same-to-same option, you need to return the car with the same amount of fuel it had at pickup. If not, you'll be charged for the missing fuel at a rate higher than the local gas stations. Alternatively, you can choose to prepay for a full tank. This means you won't need to refuel the car before returning it. However, the cost will be higher overall since you'll be charged the local gas price minus a per-gallon discount, leading to an increased total cost.