Although it is best to use the oil viscosity recommended by the manufacturer of a given vehicle, it is sometimes acceptable to substitute 5w30 oil for 5w20. This might result in a drop in fuel economy, especially during the winter months.
Every car is manufactured according to its own specifications, and manufacturers recommend a specific oil viscosity to protect the engine's internal components. Switching to a different level of viscosity might make the vehicle less efficient because oil moves differently depending on its inherent viscosity and the ambient outdoor temperature. The Society of Automotive Engineers grades oil from low viscosity to high, which means that 5w20 is less viscous than 5w30.