Just my thoughts here and they are worth what you a paying for them.
The best oil depends on among other things is the use you are put the bike to.
By that I mean if this bike is one that you only do short rides to local bike shows or the hamburg shack on sunny days and only say 1,000 miles a year and change it every fall or spring then the conventional 20W-50 should work just fine.
Now if you regularly are doing cross-country rides on the interstates and putting 30,000 miles or more a year on the bike then I would suggest the full synthetic motorcycle specific oils.
Then again if you are doing that kind of riding I would wager a guess that you are not using your XS650!
To me it would be more important to use oil that works with wet clutches and it be checked and changed often. What is "often" would depend on miles rode a year.