Oil is the lifeblood of your car's engine, so making sure it gets the right type of oil is important. With so many choices, one can easily become confused. But it doesn't have to be confusing; you just need to know how to read the label.
Most of the information you need can be found in the "API donut" that is printed on the label.
The top of the API donut
At the top of the donut, you'll find the phrase "API Service" followed by a two-letter abbreviation. API stands for American Petroleum Institute, which sets standards for motor oil quality. Each of the letters in the following two-letter code tells you something about the oil:
The first letter tells you what type of engine a motor oil was formulated for: S-type oil is for gasoline engines, and C-type oil is for diesel engines.
The second letter tells you the general quality of the oil. The higher the letter, the better the oil.
For regular gasoline engines, you'll likely find SJ, SL, and SM motor oils. If your owner's manual recommends SJ-quality oil, you can upgrade to SL or SM, but you should never downgrade to a lower-quality oil.
The middle of the API donut
In the center of the API donut is the viscosity rating. Viscosity is the measure of a fluid's thickness, or its resistance to flow. Low-viscosity fluids (like water) flow easily; high-viscosity fluids (like honey), are thick and don't run as easily.
Viscosity ratings are controlled by the Society of Automotive Engineers (SAE). For many years, a motor oil had only a single viscosity grade (or weight), and many gas-powered lawn mowers still use a single-grade oil. But the problem with single-grade oils in a car engine is that as the engine heats up, the requirements it places on the oil change.
Oil companies add polymers to its oils to address this problem, which is why you now find viscosity ratings that look like this: SAE 5W-30. This is a multigrade (or multi-weight) oil that is designed to have the viscosity of a 5W oil (the "W" stands for winter) in cold weather and a viscosity of 30 at normal engine temperatures. Multigrade oils give your engine the low viscosity it needs at startup (especially during the winter) as well as the higher viscosity it needs when you hit the highway.
The bottom of the API donut
Along the bottom of the API donut, you will either see or not see the phrase "Energy Conserving." Oils with this label (and you should stick with ones that have it) have added friction modifiers to improve fuel efficiency. Oils have to pass certain independent tests to receive the "Energy Conserving" seal.
Also on the label
The API and the International Lubricant Standardization and Approval Committee (ILSAC) have developed a certification seal that indicates that a motor oil meets the latest quality standard and the Energy Conserving standard. This "starburst" seal says "American Petroleum Institute Certified" around the outside and "For Gasoline Engines" on the inside. Look for this seal first when you're picking out your motor oil.