AMSOIL recommends using the viscosity the manufacturer recommends. But using slightly thinner or thicker oil “likely” will not cause any harm. Today’s moderns engines are built to a lot tighter tolerances than years before. This allows for the use of lower viscosity oils. It also reduces the internal friction of moving engine parts. Using a lower viscosity oil will also increase fuel mileage. Using a much thicker oil could possibly cause some engine parts not to receive lubrication due to the tight tolerances, this article provides more info.