Many users believe that if gear oil has a higher viscosity, the oil film will be thicker and have better strength, therefore more reliable, while gear oil with a lower viscosity will have a fragile and weaker oil film, and therefore a less desirable choice.
This view is partial, as the strength and anti-wear performance of the oil film is mainly depending on the quantity and quality of extreme-pressure anti-wear agent used, while the thickness of oil film only plays a secondary role. On the other hand, gear oils with high viscosity will have a greater resistance to flow, thus a low fluidity, and a lower heat conduction effect. Therefore, when choosing gear oils of equivalent qualities, the one with lower viscosity is more desirable for its fluidity. For all types of lubricating oil, adopting a lower viscosity is an industrial trend.