91 RON is a different rating system. 86 octane pump gas is the same as 91 RON. Just run 87 and you shouldn't have a problem. It really makes no difference other than the weight of your wallet - and even that difference is only marginal (as it only costs 30-40 cents more per tank to run 92 pump gas).
The difference is really just psychological - so run what makes you feel better. It makes me feel better to run synthetic oil (but that's another topic)
