I couldn't find the AAA article itself, but found this good summary of it on the Brickyard Volvo site.
Monday, 28 June 1999, at 12:24 p.m.
89 octane gas will not do any harm to your or anyone else's engine UNLESS the car pings on that grade of gas. In fact, if you are buying any higher octane than the car doesn't ping on, you are wasting your money. AAA had an article on gasoline octane and how oil companies use consumers' ignorance to sell them premium gas when in reality only very few cars on the road today require it. Basically, the jist of the article is this. The octane number is a measure of gasoline's combustobility. The higher the number, the less combustable the gas. To make the gas less combustible, impurities are added to the gas. Yes, this is counterintuitive, but the higher octane gas actually has more impurities in it to raise the octane number. Most of the time the additives are ok, but sometimes (and in most states oil copmapnies aren't required to disclose the content of the gas formula) the additives are wood- or grain-alcohol based, which can be bad for your engine in the long term. Also, in many states there is no way to know that what you get is what you paid for. In other words, surprise inspections revealed that many gas stations (even big name ones) sold regular grades at premium prices. So, AAA's advice was to buy the cheapest gas that the car doesn't ping on. I personally put 87 (regular) into my cars and my motorcycle and every few months I pour a bottle of good quality fuel injector cleaner into the tank, since regular grades usually have less cleaning agents in them. So far, I had no problems with any of my cars that were fuel related. My last car was a turbo (Mazda 626) and I fed that car regular, too. The performance is insignificantly effected, but not nearly enough to justify $0.20/gallon price difference. Premium gas was developped when many cars pinged or knocked going uphill in the older days. This occurs from pre-ignition, i.e. the fuel mixture ignites before the spark and before the valves are in the appropriate positions. The force of premature ignition opens the vales "against their will" and this produces pinging sound. If you don't hear this sound (and you shouldn't -- with today's sophisticated engine management this problem is all but elliminated), there is no harm being done to your engine. Keep in mind that regular grades typically have less detergents in them (but they don't completely lack them), so if you are worried about that, throw a bottle of fuel injector cleaner when you fill up once a couple months and save your money for other things. You Volvo, as well as most electronically managed cars on the road today will adapt to any octane in the range specified in your owner's manual (I think the lower limit is 87).