It is now possible to add Fuel data in litres or US gallons in the Journey Log panel.

In a section Settings > Fleet, edit the aircraft and set up a default fuel unit as l or USGAL

Once the default fuel unit is set up, you can add fuel in litres or US gallons in the Journey Log panel (as on screenshot below) - the default fuel unit applies to: Block fuel, Used fuel, Remaining fuel and Uplift.

The same defined fuel unit shows in the panel (suitable for adding JL data for multiple sector trip)