AMSOIL recommends using the viscosity the manufacturer recommends. But using slightly thinner or thicker oil “likely” will not cause any harm. According to this article, 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.