As Ron noted, 87 octane is the recommended fuel for the 2.5L "ecotech" I4 and that is what is was designed to use. You have found a lousy dealer which unfortunately are about as common as pigeons in NYC.
I agree with Ron to try a different fuel station and even if you have been using fuel from a Top Tier consortium member, try dosing the next couple of tanks with Techron which actually is an excellent fuel system and injector cleaner that will also remove carbon buildup in the combustion chamber. Techron was the original high detergent additive for most top tier fuel suppliers and it is effective at addressing a wide range of issues from fouled fuel tank sending units to carbon buildup in the combustion chamber and on cylinder faces. Typically pre-ignition, i.e. pinging, in an engine with some hours on it is due to carbon buildup in the combustion chamber area of the cylinder head which creates carbon hot spots that can ignite the fuel charge prematurely and sufficient carbon buildup will also reduce the volume of the combustion chamber which effectively increases the compression ratio. GM sells a pretty decent "top end" cleaner but the new formulation requires a different procedure to use which isn't consumer "do it yourself" friendly. Some swear by Seafoam and similar but if there is sufficient carbon buildup, it is worth going to a decent shop (can be an independent since you are out of warranty) who can properly use the top end cleaner. Be VERY careful experimenting with cleaning formulations because careless use can damage the expensive catalytic converters and after use of any of these get an oil change done within a few miles.
Poor quality fuel coupled with a majority of driving that keeps the engine far below its potential power level leads to faster carbon buildup in the combustion chamber. If there are carbon points formed in the chamber, they will retain a great deal of heat at the point and can ignite the fuel as it is injected. If heat related pre-ignition is occurring, then higher octane won't help. Higher octane fuel provides the desired slow burn/flame front when pinging is resulting from decreased combustion chamber volume but if the fuel is partially lighting off as it is injected by a carbon hot spot then regardless of octane rating (the ability to avoid rapid ignition under high pressure) it will still not provide the desired burn characteristic. I suspect that most direct injection engines (all of the ATS engines and most modern engines) are somewhat prone to combustion chamber carbon buildup given how much carbon "soot" specks appear on the exhaust tips compared to typical port injected engines. This isn't the same issue as intake valve buildup which is another direct injection issue that plagues some engines but is not apparent on current GM offerings.
As Ron noted, use of 87 octane fuel didn't damage your engine and when the underlying issue is addressed then it will be fine.
Rodger
