Why is quality motor oil important?

By: Patrick E. George

Image Gallery: Car Engines Frequent oil changes are a critical part of proper engine maintenance. See more pictures of engines.
Image Gallery: Car Engines Frequent oil changes are a critical part of proper engine maintenance. See more pictures of engines.

Changing your car's oil at regular intervals isn't just a good idea -- it's a vital part of keeping your car's engine running properly. The purpose of engine oil is to keep the internal parts of your car's engine lubricated and cool. It keeps the moving parts from grinding against each other causing wear and damage.

Without frequent oil changes, dirt and sludge can build up in the engine, and old, dirty oil won't lubricate the moving parts as well as new, fresh oil will. Dirty oil leads to serious damage, and if things get bad enough, there may be an engine replacement in your future.


Fortunately, changing your oil is a simple and relatively inexpensive procedure. Depending on what kind of vehicle you have, you can get your oil changed at a lube shop or dealership every 3,000 to 5,000 miles (4,828 to 8,047 kilometers). It's not hard to change the oil yourself, either.

But what are you supposed to say when you get to the counter at the lube shop and the mechanic asks you, "What kind of oil do you want?" Of course, you'll want to use the weight of oil recommended in your owner's manual, but what brand should you use? Should you choose mineral oil, a blended oil or a synthetic oil? Is there really any difference between a quality motor oil and a less expensive brand?

In this article, we'll look at why it's important to invest in good oil, and how high-quality motor oil will keep your engine running. Up next, let's talk about motor oil ratings.

Motor Oil Ratings

Be sure to look for the "starburst" and "donut" seals from the API on the bottle.
Be sure to look for the "starburst" and "donut" seals from the API on the bottle.

When you're considering a type of oil to purchase, look for the "starburst" and "donut" seals from the American Petroleum Institute (API) on the bottle. These identify that the oil meets the API's service standards, and will help keep you away from the lower quality oils.

The API's current highest rating for motor oil is SM. This oil is intended for all gasoline engines currently in use and has been the standard since 2004. For diesel engines, the current top oil rating is CI-4. In the API's words, SM oils are "designed to provide improved oxidation resistance, improved deposit protection, better wear protection, and better low-temperature performance over the life of the oil" [source: API].


Older types of motor oil (category SA thru SH) are considered obsolete but are often still on the market. Customers who don't know any better may inadvertently purchase these oils. For instance, SA oils contain no additives and could lead to higher emissions and engine damage in cars built after 1930. And category SJ protects all gasoline engines from 2001 (and earlier), so everything below the grade of SJ is considered obsolete.

In 2004, the Alliance of Automobile Manufacturers conducted a study that compared SL oil -- which at the time of the study was the top standard and is still usable today -- to SA oil in modern engines. The results were that the engines that used SA oil had more wear and more sludge than the SL engines. The SA engines had excessive deposits and wear, and were more likely to lead to catastrophic engine failure.

Now that you know why you should only purchase SJ-, SL- or SM-rated motor oils, we'll look at which types of oils are best for your vehicle.

Using Quality Motor Oils

Some car engines can benefit from synthetic oil more than others.
Some car engines can benefit from synthetic oil more than others.

Before we discuss the oil you should use, let's look at three different types of oil that you may have heard about:

  • Mineral oil is created from refined crude oil. It's the least expensive option and works just fine in most everyday vehicles.
  • Synthetic blend oils are made in a laboratory and are often designed for older and "high performance" engines. They're more expensive and offer longer life and lubrication. As the name indicates, synthetic oil is mixed with mineral oil to some degree.
  • Fully synthetic oils contain no mineral oils. They're often used in industrial applications, and have incredibly high longevity. They tend to be the most expensive oils available.

Different cars require different types of motor oil. For example, a brand new Toyota Corolla probably doesn't need fully synthetic oil, but an older Chevrolet Corvette might benefit from it. Remember, synthetic oils are more expensive, so you could end up paying for something you really don't need.


Lots of companies that create synthetic blend and fully synthetic oils advertise gains in horsepower or fuel economy. Take these ads with a grain of salt -- while they may improve efficiency somewhat, the difference may not be all that noticeable to you as a driver. However, some new oils are formulated for older, "high-mileage" engines, and these could be beneficial to your car's engine if you have a vehicle with more than 100,000 miles (160,934 kilometers).

At the end of the day, it's best to consult your owner's manual when you're choosing oil for your car's engine. Just remember to check for the API seals so you know you're getting quality oil. And while there's an endless debate over which brand of motor oil is supposedly the best, if you know how to spot a quality motor oil (and you change it at the recommended intervals), your car's engine will likely run for a long, long time.

For more information about motor oil and other related topics, follow the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • American Petroleum Institute. "API Motor Oil Guide." (May 26, 2010) http://www.aa1car.com/library/API_ratings.pdf
  • American Petroleum Institute. "Impact of Low Quality Oils on Engine Wear and Sludge Deposits." (May 26, 2010) http://www.motoroilmatters.org/download/SA vs SL comparison.pdf
  • CarBibles.com. "The Engine Oil Bible." (May 26, 2010) http://www.carbibles.com/engineoil_bible.html
  • Weissler, Paul. "How To Pick The Right Motor Oil For Your Car." PopularMechanics.com. (May 26, 2010) http://www.popularmechanics.com/cars/how-to/products/1266801