I have two R1100GS`s and used to run them both on Castrol GP 10w40 mineral oil in the engine.
When one of them got to 42,000 miles,I switched to Duckhams 'Q' 20w50 mineral and it`s running great on it.
I don`t agree with using synthetic oil ... or even semi-synthetic ... in these engines and seeing as I change the oil every 3000 miles,it would be rather expensive anyway.
Gearbox and bevel box get Hypoid 80w90 GL-5.