I just bought a 2007 Pontiac Grand Prix GXP with 67k miles. Filled it with 92 octane gas and drove 240 miles home and didn’t have one issue while traveling at a constant 70 mph. About a mile from home the check engine light started flashing and the car was idling rough. Used my scanner and discovered code P0307 cylinder 7 misfire. Could the difference in octane cause the spark plug to foul? I’m guessing the previous owner used 87 octane due to all the soot on the exhaust tips.