i kinda mislead him. I don't have a GSR, so I can't tell for sure, but if E and NoJoke say you can use 89 (and not ping) then you can use 89.
What I was saying was if you do use 93 like Milan says, then 91 would be safe as it's in the same group of premium as 93 since a lot of stations won't have more than 91 octane.
On a just thought up question...how do you figure out octane ratings? r+m/2 is the formula...but what does it mean?