The volume of the tank on your XJ is 15L.
The first warning signal appears on your dashboard when the remaining driving range is 1500 miles. A signal is displayed when the remaining driving is 900 miles. Once the tank reaches a remaining mileage of 300 miles, the warning light stays illuminated on your dashboard. A top-up of 4L when the tank is empty will turn off the warning light.
Even if the tank is empty, you can continue to drive as long as your engine is not stopped. If you stop, fill it with the contents of at least two 2L canisters.
This information can vary on some vehicles of the same model in relation to the options chosen by the manufacturer.