All higher octane does is slow down combustion in high compression engines to prevent detonation. Your Cub does not have high enough compression to require anything other than 87 octane. In some cases using higher octane than necessary can actually cause you to use more fuel. I proved this several times in my antique 1958 Harley Davidson motorcycle. It has low compression like 7:1 or lower (I don't remember offhand). When I used regular gas I consistently got 35 mpg. When I used premium I dropped to 33 mpg. I tried this enough times to eliminate other variables. My theory was that the higher octane retarded the combustion rate enough that the gas didn't complete its combustion before the piston reached the bottom of its stroke. Maybe I'm all wet but I'll swear to the mileage difference.
Bob B.