That's just what I was figuring. You can use 87 if you want to...the engine will pull the spark timing back because it detects knocking on the lower octane fuel. If you use the correct octane based on it's 10:1 compression ratio, then there will be no knocking and the engine can run optimally without slowing down the timing at all.

Pretty weird that they don't just specify 91 octane. Even thought an engine can manage itself with knock sensors, it still lets a few pings slip by and that means that over time you'll start seeing some engine damage. I would stick with 91...more performance, no chance of long term damage, and you only spend another couple hundred dollars per year anyway.