I have researched this as I plan to drain/ replace my gear oil next Spring/ Summer.
This is what I believe is the correct oil;
Part number; G 052 171 A2 (1 litre bottle) , and the cheapest I could find it was online, do a google search for "darkside developments", it's on their website.
(I can't put the link as forum rules say I can't post links until I've done 15 or more posts, themz the rulez)
I understand the capacity to be 2.2 litres.
If you only need 1L for top up, search that part number on ebay, you can get 1L for £24.99 delivered.
Search "G052171A2".