I thought I saw in the owners manual that 87 is what's recommeded.
I usually run the higher grade, just cause they say it's supposed
to be better. (But I still haven't figured out who They are ?!)
I have run 87 more than a few times, and can also say I couldn't notice much difference.
Run a few tank fulls of each, and see how you feel about it.
Here's one way to see if there's a difference ............
Try to pay attention while going up a particular hill you travel, with the bike in 5th gear, and keep it steady at whatever RPM's you feel comfortable with.(but not really flyin'... just a bit above "chuggin'" it) Then try the other grade the same way, same RPM's, driving conditions, (and preferably same temperature)
Try doing it when each tankfull is down to about Ĺ.
I would think if one works better for your bike than the other, this way might tell.
Just a though, as I'm not a trained fuel systems analyst, and I have no written documentaion to back up this hypothesis.
Which means, if it comes down to it, I will deny everything !!