In a highly competitive environment, it is essential to maximize the efficiency of hydraulic equipment to lower operating costs, conserve resources, and increase productivity. To achieve these goals, it is necessary to minimize internal pump leakage at high operating temperatures, and reduce mechanical energy losses at low start-up temperatures Cost effective hydraulic systems are designed with the smallest possible pump and fluid reservoir in order to reduce size and weight. The fluid in these modern systems must function under ever increasing temperature and pressure demands. At peak operating conditions the fluid viscosity is often too low, resulting in poor pump efficiency, inadequate flow rates, reduced load carrying capacity, and system overheating. Models have been developed to determine the volumetric efficiency for a variety of gear and vane pumps as a function of pump speed, fluid pressure and viscosity. They can be used to estimate the impact of fluid viscosity on power requirement and response time at any given temperature, and thus can help optimize system design and fluid selection.