#115.....
Ive tried all four US octanes (87, 89, 91, 93) since I too didn't have a clear understanding of what to use. The last two (91/93) are depending upon location of course. The only difference I have noticed is the strength of the gas odor in my garage after riding. The gas smell is much stronger at the higher octanes. AKI 87 produced minimal odor after a ride (if any). 91/93's odor lingered for quite some time. For the before mentioned reason, I now use only 87 and have no problems at all.