The engine oil and fuel can be refilled, and the aircraft battery recharged, from the refill menu. This menu can be accessed whilst the aircraft is parked with the engine off by clicking on the intercom switch located on the left main panel. The refill menu will appear onscreen: 

Selecting the 'Refill fuel' option will bring up the fuel menu, allowing you to set the fuel level from empty (0%) to full (100%):

The refill menu also allows you to check the current oil quantity.