It is an oil that is widely used in gasoline and light-duty diesel engines. 5w30 is a multigrade oil which means it varies from a low viscosity grade of 5 to a higher viscosity grade of 30. It is suitable for cars in cold climates, unlike oils with a higher hot temperature rating such as 10 or 15, 5w30 oil can flow freely when the engine is started at low temperatures to coat and protect internal engine components.