He obviously knows that. That is the whole reason for this thread. Lets try to make helpful, intelligent posts.
To try to answer your question, no, I don't think the lower octane fuel caused the cel. The IS has knock sensors and if knock is detected, the car should start to run a retarded map. The ecu has the ability to compensate for a lower octane fuel, however, it will most likely result in lower gas mileage and slight, if any, pinging/knocking. You can do a couple of things: First, get the cel read at a local auto store and see what it is. Second, reset the ecu by unplugging the negative side of the battery for about 10 minutes and pump the brakes a couple times while it is unplugged. Third, burn off most, if not all of that fuel and then fill up with premium.
I actually put 93 from one gas station but apparently the gas wasn't "high quality" cause it did cauase me to throw 6 codes. I drove around till i used up most of the gas. I used a scan tool and cleared the codes and now it's been 4 months and they haven't come back yet. And just one more then, people stop telling people to unplug the battery to clear the code, can we please do it the right way and clear it with a scan tool!!!!!!!!!
More than likely, it's an O2 sensor CEL at that mileage. It's probable that the 87 octane provoked an already marginal O2 sensor/heater element. 87 octane will use a retarded ignition map, causing the exhaust temp to be marginally higher.
Don't worry though. 87 octane, while not ideal, will not hurt anything and it's about the mileage that the O2 sensors need replacing anyways.
problem: wife put the cheap stuff in the tank.
solution: have wife siphon cheap stuff out of tank and put the good stuff in.
bonus: inform wife that if she can suck the gas out of that tank she can suck.... wait... happily or unhappily married?