Which property is most critical for the performance of gear oils?

The viscosity index is crucial for the performance of gear oils because it indicates how the viscosity of the oil changes with temperature. A high viscosity index means that the oil will maintain a more stable viscosity across a wide temperature range. This is particularly important in gear applications where the oil may be subjected to varying temperatures due to operational conditions, such as loads and speeds.

When the viscosity remains more consistent, it ensures adequate lubrication between gear surfaces, reducing wear, preventing overheating, and extending the life of the gears. In contrast, if the viscosity changes significantly with temperature, it could lead to insufficient lubrication at high temperatures or excessive viscosity at low temperatures, both of which can adversely affect gear performance and increase the risk of failure.

Other properties, such as base oil type, flash point, and pour point, are also important in specific contexts but do not directly impact the lubrication and temperature performance as fundamentally as the viscosity index does.