Which of the following factors could reduce the efficiency of a hydraulic system?

High fluid viscosity can significantly reduce the efficiency of a hydraulic system because it impedes the flow of hydraulic fluid through the system. When the fluid is too viscous, it requires more energy to pump, which can lead to increased power consumption and reduced system performance. This can also cause higher pressures within the system, resulting in stress on hydraulic components and possible overheating. If the viscosity is excessively high, it can prevent the hydraulic fluid from adequately lubricating the components, leading to wear and tear over time.

In contrast, options that mention clean and filtered hydraulic fluid, a proper maintenance schedule, and the use of compatible hydraulic fluids contribute positively to the efficiency of a hydraulic system. Clean fluid ensures that contaminants do not hinder flow or cause damage, while regular maintenance helps in identifying and mitigating issues before they affect performance. Using compatible hydraulic fluids guarantees that the properties of the fluid align with system requirements, promoting optimal operation.