Short answer
Kinematic viscosity is an oil’s resistance to flow under gravity, measured by timing the oil through a calibrated glass capillary under ASTM D445 and reported in centistokes (cSt), equal to mm²/s. Data sheets usually list it at 40 °C and 100 °C. SAE engine and gear grades are defined at 100 °C, while industrial ISO VG grades are defined at 40 °C.
Kinematic versus dynamic viscosity
Kinematic viscosity measures flow under the oil’s own weight. Dynamic (absolute) viscosity, in centipoise (cP), measures resistance to an applied force and is what cold-cranking and HTHS tests report. Dynamic viscosity equals kinematic viscosity multiplied by density, so the two are close for oil but not identical.
Why two temperatures
The 100 °C value represents a warm engine or gearbox and is used for SAE grades. The 40 °C value suits industrial equipment that runs cooler and is the basis of ISO viscosity grades: an ISO VG 46 oil, for example, must measure within 10% of 46 cSt at 40 °C.
Having both numbers also lets a lab calculate viscosity index, which shows how strongly the oil thins with heat.
Using cSt to compare oils
Comparing 100 °C values within one SAE grade shows whether an oil sits at the thin or thick end of its range; a 5W-30 can fall anywhere from 9.3 to just under 12.5 cSt. That is useful when two oils carry the same grade but behave differently in a particular engine.
Viscosity in used-oil analysis
Oil analysis labs track kinematic viscosity, usually at 100 °C for engine oil and 40 °C for industrial oil. A falling value can mean fuel dilution or polymer shear; a rising value can mean oxidation, soot loading or a top-up with a heavier oil. Labs compare each sample with the new oil and with earlier samples from the same machine.
Frequently asked questions
Is cSt the same as an SAE grade?
No. The SAE grade is a range defined partly by cSt at 100 °C, plus other tests. Two oils in the same grade can have different cSt values.
Why do industrial oils use 40 °C?
Many industrial systems, such as hydraulics and compressors, operate nearer that temperature, so the ISO system grades oils there.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].