First of all, I have limited experience of extended mileage XS engines, so do not have any strong opinions on this slippery subject....
But, as at least one other member points out, the oil grade used should to some degree reflect the local temperature where the bike is ridden, as well as how it is ridden. I for one, use my bikes in any temperature above freezing, and due to local speed limitis mostly stay below 60-65 mph. Someone road racing an XS in a hot climate, or even cruising the Autobahn at 80-90 mph surely need somewhat higher viscosity oil than me, living a half days ride above the Arctic Circle.
Also, say a 5w50 is not supposed to be thinner than a straight 50 oil at operating temperatures. It just has a superior viscosity index, so does not get syrupy at low temps. Sort of having the cake, and eating it too. Must be good for the sump filter, at least.
Then there is the matter of oil change intervals vs oil filter arrangement. Could it be that short recommended change intervals are due to the cleaning requirements of the oem filter? And also its limited filtering ability, leading to contaminated oil? If so, a spin on or remote filter setup should help a lot with extending safe oil change intervals. On the other hand, if the ball and roller bearings in the XS somehow break the long molecular chains in the oil and additives, then that would be the determining factor. I believe someone marketed engine oils specifically for roller/ball bearing engines. Perhaps HD, or one of the single cyl specialists, like KTM or Husqvarna.
In any case, air cooled 2 valve Ducati engines have ball main bearings, ball camshaft bearings, plain big end bearings, but little pressure on cam and rockers, due to hardly any spring pressure. And I will happily run one of those at least 5000 miles between oil changes. They do have a car sized spin on oil filter, so most likely that plays a major role.