Silkolene 20W-50
Just bought 10ltrs of this from Opie Oils for £37.99 delivered

That should last a few top-ups
It is API CE Plus / SJ which replaces the obsolete (but just as good I imagine) SF,SG or SH as recommended in my handbook for a 1150GSA (7th edition GB).
Used Motul 15W-40 before that which was fine. Again from Opie Oils.
It is in a pretty bottle for the Harley boys marketed as 'V-Twin' even showing one of their shiny engines!
I am sure if it had a BMW boxer engine on it and was marketed for large capacity 'Boxers' people who don't read the API categories would snap it up too
Too much choice that's the problem today..
Anyway, really rate Opie Oils for service and price (if using club discount), ordered yesterday, arrived this morning at 07:50.
Also nipped in to Oxford BMW yesterday to get an oil filter. £10.90 including all the crush/seal washers for the fluids (final drive, gearbox, sump). Better value than the independents I think..if you are passing.