Which of the following effects does a high viscosity index have on hydraulic fluid?

A high viscosity index indicates that a hydraulic fluid maintains a more consistent viscosity over a range of temperatures. This stability in viscosity is crucial in hydraulic systems where temperature variations can greatly affect performance.

When the viscosity index is high, the fluid's resistance to thinning as temperatures increase is enhanced. This means that even in environments where operating temperatures fluctuate, the fluid will not become too thin at high temperatures or too thick at low temperatures. Consequently, this stability supports effective lubrication and efficient operation of machinery, as it prevents issues such as cavitation and ensures that components are well-lubricated under varying thermal conditions.

The other options relate to different characteristics of hydraulic fluids that do not directly stem from a high viscosity index. For instance, while thermal expansion occurs in fluids as they heat up, a high viscosity index itself focuses more on maintaining optimal viscosity rather than allowing for greater expansion. Similarly, high viscosity does not inherently reduce lubrication properties or increase flammability—these factors depend on other chemical properties and formulations of the fluid.