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.
Go to the next page to learn how viscosity is measured.