Im not sure how gasoline is rated in Japan. If an octane rating is available then you should
get gasoline rated in the high 80's or better low 90's.
Try regular and premium to see if it affects your bike's performance. I did the same in the US and did not notice any difference.
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Ride safe and ride within your ability but most of all .....have a frickin blast!!
I almost always try to get mine on base, but sometimes I'm just into the ride that I don't want to head all the way back for gas (my wallet sure hates me though!)
Dana
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'01 996S Homologation Model (Euro model) Yellow