Also did some reading just now on 'what oil to use for RoskShox' and found loads of people advising to just go to motorbike shop and buy the suspension fluid there - much cheaper (especially if you don't need a litre of the stuff.
So popped to my local motorbike store and bought some 5wt there, 500ml for £7
Also ready recommendations to use 5w -40 fully synth motor oil in the lowers oil-baths instead of the 15wt as the motor oil lubes better and sticks to the components better too.. I happen to have an open 1 litre bottle of 5w-30 fully synth Shell Helix in my office from my Touareg so I'm gonna give this a go