Way back when in the 1970's when unleaded gas was introduced, so was the 89 octane. This is because unleaded gas builds deposits on the cylinder resulting in preignition occurring after around 20,000 miles. With 87, the engine used to ping from these hot deposits and so 89 was provided to stop it. Today's modern engines have an antiknock sensor that retards the timing so you don't notice it, but if you want to retain the original performance, you must either add stuff like Techroline every few 1000 miles or use 89 gas. Additionally, some cars will advance the timing if higher octane gas is used thereby providing a few more horsepower. My experience is that premium adds about 7% to the gas mileage and so if the price difference is less than or equal to this, go for it. Unfortunately, the local Costco does not stock 89 so I must alternate between 87 and 91 to get the blend in the tank. Kinda awkward but it works.
As to hurting the engine, no, higher octane does not.
Hope this discussion helps.
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