For example, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage up to 2%. The thicker the oil, the harder it will be for the pump. This adds to parasitic horsepower loses.
You got it backwards, 5w-30 is thinner than 10w-30. Also the 5w is for when it's cold - the 30 is for when engine is warmed up. I wouldn't recommend using oil that the manual doesn't call for due to damaging the engine which would negate the savings in gas, however if you are wanting to save gas, you want the Xw-20 or lower.
$3.69 at Shell.