Mate,
As per the above posters...Six Lt of oil consumed per 6,000 (1Lt/1000 miles) is considered normal for many performance cars.
It’s a bit of a taboo subject. Many owners don’t want to admit this issue and for various reasons are in denial... But in the grand scheme of things don’t get too concerned, you can get premium quality oil such as Shell Ultra 5w30 or try 5w40 for less than £25 for 5L at Eurocarparts.... Just be vigilant and top-up your levels accordingly and enjoy your Z !