Don't worry about your 5W30 oil. I doubt if the change from 5W20 was responsible for the drop in gas mileage. The only accurate way to compare mileage is:
- Use the same gas at the same station AND THE SAME PUMP! Stop at the first click each time! Don't try to top up the tank. Beware that each pump is different.
- Only compare the same kind of driving patterns; city, highway or mixed.
For instance with my Toyota I have had gas mileage range from 45mpg to 32mpg reflecting different driving styles.
Hondas in foreign countries use different oil in the same engine. In the tropics my neighbour's Honda required 20W50!!!Only the US has mandatory EPA CAFE mileage standards with penalties. Therefore US built cars are pushed into the thinnest possible oil resulting in the highest mileage, but at substantially shorter engine life.
A few weeks ago we had a simlar posting, and various respondents commented that a) modern engines have tighter clearances, b)modern oils are better quality. However, the best way to ruin your engine is to fill it with cheap 5W20 oil, and set out for Death valley in July. If you really want to use 5W20, go synthetic, Mobil 1 or similar quality. North American manufacturers balance warranty costs due to very cold starts and quickly loading the engine with possible fuel penalties if they sell too may big cars.
So the 5W20 decision has little to do with well being of your car and everything with the manufacturer's bottom line profitabilty. If I was in your shoes I would use Mobil 1 synthetic 5W20 until the engine warranty was up, and then switch to 0W30 (yes, there is such a thing)synthetic in the winter and 5W30
synthetic in the summer.