If you want the best gas mileage, an xW20 oil will be best, the 20 weight oils are a little harder to find at discount. I think the Motorcraft 5W20 Syn blend at Walmart is one of the cheapest I have seen and will easily take you to a 5K oci. I think I have seen Pennzoil 5W20 at Walmart as well.
I have had great results with Mobil 1 0w20 in my Prius. Bill in Utah gets great results with Castrol GTX 5W30 in his 2005 Corolla (3 ppm Fe in 5.8k miles, during Jan and Feb no less!). He drives a lot of highway miles and gets great gas mileage so I don't think he stomps on the gas pedal all the time.
Any SM oil will be fine, I think you may get a little better gas mileage with M1 0W20, but it will cost an extra $20 per oil change. If you got 30 mpg over 5k miles at $2.75/gal the gas would cost about $458. Lets say Mobil 1 0w20 results in 31 mpg over 5k miles, at the same gas price your total gas cost would now be about $444 so it would be almost a wash in total cost, but if it were my car I would choose the synthetic for $6 extra. Many would argue that the synthetic does not give you better gas mileage than the dino, I have no proof either way because lately I have been using only synthetic. Bill in Utah has tried both and saw no fuel economy difference and better UOA results using dino. If he is right about the fuel economy, then dino will save you money, especially if you look for closeouts and other specials and get your oil for 50 to 70 cents per quart. I doubt there are a lot of great deals on 20 weight oil, I think you'll have to pay at least $10 for a 5 quart bottle. Good luck.
Dennis