I usually fill up at Shell, but last Friday filled up at the Circle K because it had a cheaper premium gas price. I actually did not notice it was a Circle K, but instead saw the 76 sign, and thought it was safe. The next morning, started the car and it threw up the Check engine light. Engine felt bogged down, timing retarded, rough idling and almost stalling. I reset the CEL and this morning I added half a tank of premium gas from a Shell station and car immediately felt like it had more power, stronger idle. I believe the gas from the Circle K was poor quality or mixed with water.