vauxhall

Apologies if I have the wrong space for this problem I have. I have the lexsus 1zufe 4.0 L v8l . I have the automatic overdrive transmission. I have no gearbox leaks but loose all my oil then have to change gears manually. Once I fill up I have gears. The oil is clear sweet smelling not burnt and the car is serviced regularly every 10 000 kilometres. The gear box is silky smooth up changes and down changes engaging Park or Drive or Reverse. This happens once a month out of the blue that I just loose all my oil and not a drop of oil under the chassis of the car at all . Please help I am confused.

Regards
Allan