engine oil

Photo by Robert Couse-Baker / Flickr / CC BY 2.0

How to Choose Motor Oil for Your Car

The care and maintenance of your car or truck starts with a routine oil check and regular changes. Your engine needs clean oil to prevent wear and tear on engine components, which can decrease engine performance and significantly reduce the life of your engine. The controlled detonations inside a vehicle’s engine heat the metal parts up to 600 degrees Fahrenheit. You need clean oil to remove excess engine heat and to prevent carbon and varnish buildup in the engine.

As science and technologies have evolved to produce different types of engines, so too have specific engine oils evolved to address the needs of those different engines.

Engine oils are typically classified by viscosity, the oil grade and type of oil. The sheer number of options may seem bewildering, but using the most appropriate oil for your vehicle is the best way to get peak performance and a long engine life.

Viscosity: Why It’s Important?

Photo by Jeff Wilcox / Flickr / CC BY 2.0

Photo by Jeff Wilcox / Flickr / CC BY 2.0

Oil viscosity informs you about the oil’s resistance to thickening in cold temperatures and the oil’s resistance to thinning in hot temperatures. Thick, sluggish oil in cold weather is as bad for your engine as thin, watery oil is in hot weather. Both increase friction, which can result in costly engine repairs.

The Society of Automotive Engineers – SAE – grades the viscosity of engine oils. SAE viscosity is expressed by a numerical grade such as 10W40 or 5W30. The “W” is for “winter,” not “weight.” The number preceding the W indicates the oil’s viscosity in winter. The lower the number preceding the W, the less resistant the oil is to thickening during winter. Thus, a 5W oil flows better in winter than a 10W oil.

Use the recommended viscosity range from the owner’s manual to select the best viscosity for the weather conditions where you drive.

The number coming after the W indicates the oil’s resistance to thinning at the water boiling point temperature of 212 degrees Fahrenheit. The lower the number after the W, the less resistant the oil to thinning in hot weather. Thus, a 10W40 oil is more resistant to thinning in hot weather than a 5W30 oil.

Oil Grades: What Do They Mean?

SAE grades the viscosity of engine oils; the American Petroleum Institute – API – grades the quality of engine oils for their suitability in gasoline-powered and diesel-powered engines.

A quick glance at the API service mark, called the “donut” and found on most engine oil product containers, informs you of three useful nuggets of information. The top half of the donut says that the oil is suitable for gasoline engines with the “S” designation for “service categories,” or with a “C” for diesel engines, which stands for “commercial categories.”

The letter following the S for gasoline engines designates the suitability of the oil for various types of engines. API currently uses four different letters – N, M, L, J, which designates an oil’s compatibility with an engine’s unique requirements based on the engine’s year of manufacture.

Likewise, the alpha-numeric symbols following the C for diesel engines designate an oil’s compatibility with a range of different performance-level diesel engines. API currently uses three performance-level designations following the C – CJ-4, CI-4, CH-4.

Some oils are dual-purpose that meet the requirements of gasoline and diesel engines. These oils get both the S and the C designations with the C preceding the S.

API recommends you follow your owner’s guide recommendation for the oil performance rating that’s best for your vehicle.

The center of the API donut shows the SAE oil viscosity rating.

The bottom portion of the API donut shows whether the oil meets API’s requirement for a “Resource Conserving” or “Energy Conserving” oil. Some diesel oils show a CI-4 Plus designation at the bottom of the donut. CI-4 Plus indicates CI-4 oils have a higher performance level.

Avoid using cheap oils for your vehicle that are not API graded. Low-quality oils reduce engine efficiency and do not provide the engine protection you need for a long-life engine. Moreover, you risk voiding your warranty by using non API-graded oil. It’s prudent to also avoid using various engine additives, which might do more harm than good.

Types of Engine Oils: Why Are There So Many?

types of engine oil

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Engine oils are formulated using two primary components; base oils and additives. Base oils make up 70 to 85 percent of the formulation; additives make up the remainder. You can choose from many various types of engine oils:

  • Conventional Oil
  • Premium Conventional Oil
  • Full-Synthetic Oil
  • Synthetic-Blend Oil
  • High-Mileage Oil

Conventional oil is the least expensive and is typically used by quick-serve oil shops unless you specify an oil-upgrade. If you’re a stickler for frequent oil changes and have a low-mileage engine that’s well-broken in, this may be the oil for you. These “basic” oils conform to API and SAE standards, but include fewer additives in their formulations. A new car may get more benefit from using premium conventional oil, which includes more additives than conventional oil for longer engine life.

Full-synthetic oils are laboratory creations made of chemically modified petroleum or oils synthesized from other raw materials and include high-performance additive packages. They’re specifically designed for high-tech engines.

Synthetic oils maintain peak lubricating performance in all weather conditions, which makes them a superb choice.

The downside is that synthetic oils cost up to three times the cost of conventional oils and they may not be needed for your specific engine and driving style. Let you owner’s manual be your guide, and seek additional advice from your car dealership.

High-mileage oils include seal condition additives to increase the flexibility of aging internal engine seals in vehicles that have high miles.

Additives In Engine Oil: What Do They Do?

Engine-oil additives generally fall into the following classes:

  • Dispersants keep sludge suspended in the oil until the next oil change, rather than accumulating on internal engine parts.
  • Detergents help prevent engine deposits from accumulating on high-temperature surfaces, such as around the pistons. They dissipate over the oil’s life between oil changes.
  • Anti-Wear Agents form a protective coating on metal engine parts and help prevent oxidation.
  • Friction Modifiers reduce friction caused by heavy loads and high temperatures, and diminish over the life of the engine oil.
  • Anti-Foam Additives prevent foam-causing air bubbles in the oil that retard the smooth flow of the oil to engine parts and inhibit the oil’s ability to cool the engine.
  • Viscosity Index Improvers help to maintain the oil’s viscosity over a wide range of temperature fluctuations.
  • Pour-Point Depressants found in multi-grade oils help the oil flow at low temperatures.

What Type of Oil Is Best for My Car?


Despite the hype, Consumer Report says the brand of oil matters little. The key metric that should guide your oil selection is the oil’s viscosity grade, such as 10W30, and its quality. The wrong grade of oil can wreak havoc on the life of your engine.

In order to know which engine oil is best for your car, check the oil filler cap. The oil specification can often be found around the oil filler cap under the hood. If it’s not there, consult your owner’s manual for the correct viscosity grade for your vehicle.

Make it a practice to check your oil level frequently. If you’re unsure about the recommended frequency of oil changes, consult with a professional auto mechanic or your dealership.

What type of oil do you use? Do you have a brand preference or are you more interested in its viscosity rating?

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