Many people mistakenly consider viscosity as a quality parameter. That's not the case. The viscosity of a motor oil is a physical property indicating the ideal temperature range within which the product can perform. If an oil maintains its viscosity parameters well in both cold and hot conditions during usage, then we can consider it of high quality. When the engine overheats one of the reasons may be the oil: it might have the wrong viscosity, being too fluid, or, if the viscosity is correct, the oil may has rapidly lost its properties due to poor quality.