Steve
I would actually take the opposite route recommended by Randy (no offense, Randy). I would start with low octane and only go up if you have detonation problems. Unless your engine has a high compression ratio (say, 11:1 or higher) or forced induction, you probably won't need high octane. Unless you have detonation problems with low octane fuel, you're just wasting money with high octane. I run low octane in my 12:1 Harley and my dad runs low octane in his supercharged Bonneville. In both cases, high octane is recommended or "required" by the manufactuer, but neither has problems on low octane. Then again, neither is being raced.
Randy is on track about the spec's. Find out what you have. You will eventually need to know.
Eric