Understanding Motor Oil Ratings

Motor oil ratings cover the type, viscosity and service performance of the various oils on the market, allowing you to select the proper oil for your engine. Although motor oil ratings may seem confusing, navigating them correctly will keep your car running smoothly.

Motor Oil Ratings Explained

Two groups oversee the certification of motor oils: the Society of Automotive Engineers (SAE) and the American Petroleum Institute (API). These two bodies provide standards and guidelines for oil companies who market motor oil. Here's how to decipher the ratings:

• Viscosity. Viscosity (sometimes called weight) defines the ability of oil to flow at certain temperatures. The viscosity ratings are determined by the SAE and range from 0 to 50, where 0 flows most easily and 50 is the thickest. Many oils have a "W" after their viscosity, which refers to winter-grade motor oil.
• Multi-viscosity. Lightweight oils are better for cold climates and heavyweight oils are better for hot climates. But most areas of the country experience both conditions, so motor oil companies have combined both types of oils to create oil with a wider range of performance. These multi-viscosity oils list both types of viscosity, as in 5W-20 or 10W-30.
• Service with a smile. API service ratings indicate the appropriate use and characteristics of motor oil. Oils appropriate for gasoline engine use have a two-letter rating that begins with an "S". Current ratings are "SJ" (for 2001 vehicles and older) and "SL" (for all vehicles currently in production). Older ratings are considered obsolete and are appropriate only for antique cars. Oils intended for diesel engine use a two-letter rating that starts with "C". There are many different diesel ratings-consulting your owner's manual is the best way to make sure a specific diesel-rated oil is appropriate for your engine.
• Green oil? API ratings also indicate oils that meet certain energy saving and efficiency criteria. These oils are given an "Energy Conserving" label.