Holden Commodore: Oil evaporating

Since new I have had to check the oil level in my 2010 Holden Commodore VE SV6 weekly because it is evaporating and the mechanics at my dealership cannot explain to me why it's happening. I have had my own mechanic explain to me that this should not be occurring on such a new car and one that has only done a small number of kilometres. A 2010 model Holden Commodore with 26,000 km on the clock should not under any circumstances be needing at least 8 litres of oil in the motor because it's evaporating.