Lubricity – the fluid must keep friction low and maintain an adequate film between moving parts to prevent wear of pumps, bearings, vanes, gears, pistons and rods. Increasing pressures and, consequently, closer tolerances, make lubricity even more important.
Viscosity – fluid “thickness” or resistance to flow. Pump manufacturers specify this according to clearances, speeds, temperatures and suction characteristics. The fluid must be thin enough to flow freely, yet heavy enough to prevent wear and leakage. Viscosity might not be so critical in selecting a hydraulic fluid except that it varies with temperature. Fluid thickens when it cools, thins as it heats up. Because some hydraulic systems must work under wide temperature extremes, viscosity range is important.
Viscosity Index – This measures the rate of viscosity change with temperature: the higher the index, the more stable the viscosity as temperature varies.