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 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.
Go to the next page to learn how viscosity is measured.
Measuring Motor Oil Viscosity
The standard unit used to measure viscosity is the centistoke (cSt). According to the Automotive and Industrial Lubricants Glossary of Terms:
The centistoke rating is converted into the SAE weight designation using a chart like the one shown on the Superior Lubricants Web site.
Multi-weight oils (such as 10W-30) 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 of the oil at a cold temperature, while the second number indicates the viscosity at operating temperature. This page from the Sci.Electronics.Repair FAQ offers the following very interesting description of how the polymers work: