Viscosity Units

• cSt (centistokes): Kinematic viscosity - most common for hydraulic oils
• SUS (Saybolt Universal Seconds): Older US standard, still used in some specs
• cP (centipoise): Dynamic viscosity = cSt x density (g/ml)
• mm2/s: Same as cSt (SI unit)

ISO Viscosity Grades

ISO VG grades are based on kinematic viscosity at 40C:

• ISO VG 32: 28.8-35.2 cSt @ 40C - Light duty, cold climate
• ISO VG 46: 41.4-50.6 cSt @ 40C - General purpose
• ISO VG 68: 61.2-74.8 cSt @ 40C - Heavy duty, warm climate
• ISO VG 100: 90-110 cSt @ 40C - High load, high temp

Temperature Effects

Viscosity decreases as temperature increases. The relationship follows the Walther equation (ASTM D341):

log log(v + 0.7) = A - B x log(T)

Where v is viscosity in cSt and T is temperature in Kelvin.

Optimal Viscosity Range

• Minimum: 10-13 cSt (pump wear limit)
• Optimal: 16-36 cSt (best efficiency)
• Maximum: 100-1000 cSt (depends on pump type)