Roady - different people prefer different oils but I use Morris Lubricants - which the factory recommend.
Axle oil is Lodexol 80W140 with friction modifier included.
If necessary I add a slosh of extra friction modifier sourced from Demon Tweeks.

In my early days I used semi-synthetic diff oil as recommended in the handbook, but realistically fully synthetic is arguably better.
http://www.morrislubricants.co.uk/products/auto/gear-transmission-oils/lodexol-xfs-80w-140.html

Other brands are available.
http://www.castrol.com/en_au/australia/products/cars/axle-and-universal-fluids/syntrax-80w-140.html

http://www.redlineoil.com/product.aspx?pid=37&pcid=4

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4