How does lubricant viscosity affect friction under fluid-film conditions?

Under fluid-film conditions, where a lubricant completely separates sliding surfaces, friction is directly proportional to the viscosity of the fluid.1

In this state, the lubricant's viscous properties are essential for maintaining the film that prevents solid-to-solid contact and reduces wear. Because friction increases as viscosity increases, selecting a lubricant requires balancing the need for a sufficiently thick film to support the load against the desire to minimize frictional resistance.231

While viscosity is the primary determinant of friction under fluid-film conditions, it becomes less significant under boundary lubrication conditions, where the chemical nature of the lubricant plays a more critical role. Additionally, because the viscosity of most liquids fluctuates with temperature, lubricants are often evaluated using a viscosity index to ensure stable performance and consistent friction levels across varying operating temperatures.13