Bought my z650 brand new and I used 87octane for the first 1000km.
Then for a while the bike felt strange. It had a weird response and made a couple of pings when I eventually stalled the bike. Heard the ping three other times as the bike stalled.
I figured it was engine knock, probably from a wierd spout of the engine break-in process creating some hot spots of friction.
So I switched over to 91 octane and it was night and day. Bike felt peppier, got much better mileage than the the previous tanks fillups and never again has that sound reappeared.
Now my bike has close to 5000km and I just switched back to regular octane. One tank through and I felt little to no difference. Gonna keep at it for a while and see if something changes, but I have a feeling that what ever issue I had during the break in has worked itself through.