ok guys..well after reading this guide and doing some more digging....
I came to the conclusion that oil (like many other products) cannot be judged by its brand.
eg. Castrol GTX is not necessarily good because they have loads of adverts and a strong brand name (though this is just an example )
The only problem this leaves me with..is finding a reliable supplier of Royal Purple Oils : in the UK...as its only distributed in the US it seems :(