TERM-VISCOSITY
Oil Viscosity
Lubrication
Also: kinematic viscosity · oil grade · SAE grade · ISO VG
Abbrev: cSt · cP
Canonical Definition
A measure of a fluid's resistance to flow, expressed as kinematic viscosity in centistokes (cSt) at a defined temperature, or as dynamic (absolute) viscosity in centipoise (cP). Engine and hydraulic oils are classified by SAE J300 (engine) or ISO VG (hydraulic) viscosity grades. Viscosity is the most critical physical property of a lubricant: too low and the oil film cannot support the bearing load (leading to metal contact and wear); too high and the oil creates excessive churning losses, heat generation, and pump cavitation at cold start. Viscosity decreases with temperature and increases with pressure.
Engineering Context
Oil viscosity must remain within specification throughout the service interval. Excessive fuel dilution, thermal degradation, contamination by different viscosity-grade oil, and shear loss from high mechanical stress all reduce viscosity in service. Oxidation and soot loading increase viscosity. Oil analysis viscosity measurement (at 40°C and 100°C) detects both modes and is a primary condition monitoring trigger for oil change decisions.
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| Permanent ID | TERM-VISCOSITY |
| Category | Lubrication |
| Status | Published |
| Version | 1.0 |
| Last Reviewed | 2026-07-05 |