I just recently got a 2005 Volvo S40 with a turbo engine; I want to take good care of it, so naturally, when the instruction manual recommends 91 octane fuel but says it takes a minimum of 87, I have no problem with buying 91, knowing (or at least believing) that it's better for my engine and improves performance and fuel efficiency compared to just shoving regular unleaded into its tank. I checked online, read a bit about engine knocking and whatnot, and was determined that's what I'd do, and the previous seller also filled it with premium gas so it has been well taken care of thus far.
This morning I stopped at a local gas station to refill it for the first time. My fuel octane choices were 87, 89, or 93. Oh no! I probably should have hopped back in my car and went looking for a "normal" gas station, with premium at 91 octane... But instead I just choice 89 and made a mental note to ask this question.
Should I have chosen 93? Or would that have been "too much"? Is it better to choose a bit higher or a bit lower? Could one be more harmful than the other to my engine?
I'm in the USA if it matters.