Three possibilities, your oil pressure sending unit could be leaking or your fuel pump if it's the mechanical one on the head. The fuel pump has a pressure equalizer hole in it, if the diaphram shaft seal is shot it will blow oil out while running as will the sending unit. I would expect to see some oil on the bottom of the car but you would be loosing most of it while driving. Also you could be pumping it into the cooling system, check the rad for oil in the coolant, It seems unlikely that you could hide 2 litres of oil in the rad without coolant leaking out and showing up on the ground but again it would do most of the leaking while running. How fast does the 2 litres disappear?