Generally, oils of lower viscosity will help to improve fuel economy because the moving parts in an engine will not have to overcome the internal resistance of a thicker oil.
This would mean that more energy could be used to drive the wheels or, alternatively, less fuel would be required to get to your destination. This is only part of the story, because although a thinner oil will offer less resistance to movement of the engine parts,
you need to have an oil that will be thick enough to prevent the moving metal surfaces from touching when the oil gets hot.
