According to my Gy6 scooter manual, you're supposed to use 91 octane or higher. I haven't actually noticed a difference with lower octane like 87 though. The funny thing about scooter manuals is they constantly contradict themselves. My 125cc GY6 "requires" 91 octane, but the compression ratio is 9.5 to 1, which means 85 is ok too.
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