Oil has come a long way in the last 100 years, I think you would be hard pressed to find some not up to the job, especially on a road bike.
If I was racing a superbike I may pay more, but the £60 a pop stuff your BMW dealer insists upon is just a way for them to make even bigger profits.
The biggest problem with motorcycle engines is contamination from the clutch plates (not an issue on the Boxer as it has a dry cltuch) and the gearbox destroying the Long Chain Molecules in Oils (also not a problem on the Boxer as the Gearbox has its own oil)
I would have thought the GS motor is probably one of the best in motorcycling for not requiring exotic oils and short drain intervals.
If triumph can get 12k from their higher tuned, higher revving, wet clutch with integral gearbox motor I can't see why the GS motor would not do twice as much even on cheap oil - apart from the dealer not taking £400 a year off you in servicing.
I stocked up on this oil during the closing down sale and will be using it once my warranty is up.