Oil Grading 101
There's more to oil than just viscosity
by Leif Oskarsson
Probably most folks who do their own oil changes are at least somewhat familiar with the oil viscosity ratings. What may be less understood, however, are the service grades. Just as it's important to pour the correct viscosity oil into your car's engine, it's also important that oil have a service grade that's appropriate to the engine's needs.

What's Correct?
How do you know what's correct? First, check your vehicle's owner's manual for the section on viscosity and grade. You should find a reference to something called "API," which stands for American Petroleum Institute. The API sanctions an Engine Oil Licensing and Certification System (EOLCS), which authorizes engine oil marketers who meet the standards to use certain API Engine Oil Quality Marks. These marks are the API Certification Mark "Starburst," and the API Service Symbol, known as the "donut." These marks will likely show up in your owner's manual, and they will also be on every container of engine oil that meets the API standards.

API Certified
The API Starburst identifies engine oils recommended for a certain application, such as "For Gasoline Engines." To carry this symbol on the container, the oil must satisfy the most current requirements of the International Lubrication Standardization and Approval Committee minimum performance standard for that application.

The API "donut" is divided into three parts. The top half-circle lists the API service rating, or performance level. The center of the circle is the viscosity. And the lower half-circle indicates whether the oil has demonstrated certain energy-conserving properties.

Service Ratings
The service rating indicates that oil's performance properties, such as its ability to deal with extremes of heat, to minimize harmful deposits or contribute to reduced oil consumption. The ratings are expressed as a pair of letters, and for gasoline engines the first letter is "S," for Service. The second letter—beginning with "A" and continuing to the current classification, "M," indicates the performance rating.

It is significant to remember that each succeeding, or higher, performance rating includes all the capabilities of all those that have gone before it. Therefore, the current rating, "M," will work for any automobile engine on the road. And also remember, the performance rating is not related to the viscosity. One has nothing to do with the other. By the way, there are no categories for "SI" or "SK."

Performance Ratings
The following Category/Status/Service list indicates the performance ratings:

SM — Current — For all automotive engines presently in use.

SL — Current — For 2004 and older automotive engines.

SJ — Current — For 2001 and older automotive engines.

SH — Obsolete — For 1996 and older engines.

SG — Obsolete — For 1993 and older engines.

SF — Obsolete — For 1988 and older engines.

SE — Obsolete — For 1979 and older engines.

SD — Obsolete — For 1971 and older engines.

SC — Obsolete — For 1967 and older engines.

SB — Obsolete — For older engines.

SA — Obsolete — For older engines.

The two oldest categories, "SB" and "SA," should be used only when specifically recommended by the manufacturer. Realistically, if you go shopping at the local auto parts store, you will probably find only oil with the latest ratings. And for those of you with diesel engines, there is a separate rating system, starting with the letter "C." The diesel ratings are more involved, so it's best to consult your owner's manual or operating manual to make sure you have the correct oil in the crankcase.