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Oil Viscosities 101

Jennie McRae - Monday, May 18, 2015



Ever wondered what those numbers mean on your oil can? And have you ever wanted to know what the “W” stands for, anyway?

You’re in the right place!


Today, we’re talking about oil viscosities and what they mean to you.


On every bottle of motor oil, there is a seal that gives you three pieces of information: The API service rating, the viscosity rating, and the “Energy Conserving” indicator.


The API Service rating is going to tell you the type of engine the oil is meant for (gasoline or diesel). It will also tell you the quality level.

The viscosity grade (for example, 5w30), tells you the oil’s thickness, or viscosity. A thin oil has a lower number, and therefore will flow more easily, whereas a thicker oil has a higher number. Think of the difference between water and honey. Honey has a higher viscosity than water. 10w30 oil is thicker than 5w20.

So how is viscosity measured?

Viscosity is measured in centistokes, and according to the Automotive and Industrial Lubricants Glossary of Terms:


Viscosity is ordinarily expressed in terms of the time required for a standard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is now commonly reported in centistokes (cSt), measured at either 40 degrees Celsius or 100 degrees Celsius.


This centistoke rating is then converted into the SAE weight designation. (visit: superiorlubricants.com/classtable.html to see this chart).


Multi-weight oils (such as 5w30, 15w40, etc.) are a new invention made possible by adding polymers to oil. The polymers allow the oil to have different weights at different temperatures. The first number indicates the viscosity at a cold temperature, and the second number indicates the viscosity at operating temperature.

Ever wonder what the “W” stands for?


It stands for winter! 


This is used in the weight classification of the oil to indicate that the first number gives a picture of what the viscosity will be in the winter.

So how do you know what weight of oil to use in your vehicle? Always use what your OEM (Original Equipment Manufacturer) recommends.


For more information on different oil weights, and what is best for you, please call one of our lubricant experts at 1-800-883-8081.


Sources: shell.com; auto.howstuffworks.com

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