Viscosity Grade of the Oil

Viscosity Grade of the Oil

The ISO viscosity classification is recommended for industrial applications. The reference temperature of 40 °C represents the operating temperature in machinery. Each subsequent Viscosity grade (VG) within the classification has approximately a 50% higher viscosity, whereas the minimum en maximum values of each grade ranges ±10% from the mid point. For example, ISO VG 22 refers to a viscosity grade of 22 cSt ± 10% at 40°C. The viscosity at different temperatures can be calculated using the viscosity at 40°C and the viscosity index (VI), which represents the temperature dependency of the lubricant.


SAE Viscosity Grades for Engine Oils1

The actual viscosity grade of a lubricant is determined by the Society of Automotive Engineers, for example SAE-15W40 for a multigrade oil and SAE-40 for a monograde oil. The first number (15W) refers to the viscosity grade at low temperatures (W from winter), whereas the second number (40) refers to the viscosity grade at high temperature.


Why we need to know?

  • On every bottle of motor oil there is a seal that gives you three pieces of information:
    - The API service rating
    - The viscosity grade
    - “Energy Conserving” indicator (it either is or it isn’t)
  • The API service rating is a two-letter rating that tells you the type of engine the oil is meant for (gasoline or diesel) and the quality level.
  • The viscosity grade (for example, 5W-30) tells you the oil’s thickness, or viscosity. A thin oil has a lower number and flows more easily, while thick oils have a higher number and are more resistant to flow. Water has a very low viscosity — it is thin and flows easily. Honey has a very high viscosity — it is thick and gooey