Motor Oil: What Do the Numbers Really Mean?

“220. 221. Whatever it takes.”

That infamous line of reasoning worked for Jack Butler (Michael Keaton) in the 1983 movie Mr. Momso it should work for you, too, when it comes to selecting the right motor oil grade, right? Simply pick a number? Nope! Just like with electricity, when it comes to car oil, numbers matter—especially if you want to protect your engine.

motor oil 1The skinny on engine oil weights

Oil “weights” or grades—such as 10W-30—are actually a numerical coding system developed by the Society of Automotive Engineers (SAE) to grade oils based on their viscosity.

Viscosity is measured by the how long it takes a specific amount of oil to flow through a specific-sized opening at a specific temperature. The longer the oil takes to flow through, the higher the viscosity. The tool used to conduct that test is a viscometer.

Think of pouring pancake syrup from the bottle—at warmer temperatures, the syrup pours fast and easy, while at colder temperatures, it’s thicker and more difficult to get flowing. The same can be said for engine oil.

The particular challenge with motor oil, however, is that automotive engines need engine oil to be both thin and free flowing when temperatures are freezing and the engine is cold, but thick when it’s hot out and the engine has reached operating temperature. That’s where multi-weight or multi-grade oils enter the picture and why they were created.

What does the “W” mean in oil weights?

American Petroleum Institute

SAE’s J300 standard, first published in 1911 and revised numerous times since, classifies oil into 11 viscosity grades—0W, 5W, 10W, 15W, 20W, 25W, 20, 30, 40, 50, and 60—with the “W” signifying “winter,” not weight. Oils first received this “W” designation from SAE in the 1950s. The lower the number preceding the W, the lower the temperature for which the oil is rated.

Those winter numbers were modified further after a rash of catastrophic engine failures in the early 1980s. Unusually cold weather in the U.S. and Europe caused oil to gel. When this occurred, the engine would still start, but it couldn’t pull the gelled oil out of the oil pan, resulting in the failures. As a result, SAE added a low-temperature test to measure pumping viscosity as well, and indicated this oil with the W specification.

The importance of temperature

Back to the idea of multi-weight oils. A popular oil, such as 10W-30, performs like two oils in one when it comes to engine lubrication. At colder temperatures it is and delivers a 10W-grade oil performance, while at higher temperatures it is and performs like a 30-grade oil—according to SAE’s standards and tests—providing engine protection at both ends of the temperature spectrum, which is important since engines have to operate in a range of temperatures.

Think of it this way—that SAE 30 oil you might use in your riding mower has the same viscosity as the 10W-30 oil in your vehicle, but only at 210°, the maximum temperature that SAE requires. The difference arises at colder temperatures where the SAE 30 oil can’t perform, necessitating some enhancements that make it a multi-grade oil. At those lower temperatures, that’s where the 10W oil and its characteristics come into play.

Oil’s desired performance characteristics at varied temperatures, as specified by SAE, are achieved through the addition of Viscosity improvers (VI) or modifiers that increase the oil’s viscosity as temperatures rise. The result is oil that performs and provides engine lubrication no matter what the temperature.

Know what to look for

The good news for drivers is that they don’t need to be an engineer or chemist to know which car oil to use, and they don’t have to change their oil grade whenever the temperature changes. Simply follow the motor oil grade recommended by the vehicle manufacturer for optimal engine protection in all types of weather.

oil sealIt’s important to note that SAE also has a coding system for gear oil, such that used in a manual transmission, and that it’s different than the ratings for engine oil. So if there’s a bottle of 85W-140 oil sitting on the barn or garage shelf gathering dust, don’t put it in your engine.

And finally, when choosing an oil, look for one with the American Petroleum Institute “donut” seal on the bottle. It indicates that the oil meets API performance standards.

Checking Your Vehicle’s Essential Fluids and Hoses Provides Peace of Mind

hood up fixing car

Benjamin Franklin once said, “An ounce of prevention is worth a pound of cure.” The same is true for your vehicle, especially when it comes to its essential fluids and hoses. Regular maintenance checks of these items will contribute to the sustained health of your vehicle and reduce your risk of unpleasant automotive surprises down the road. But where to start? Here are a few things to keep in mind.

Essential fluids

High operating temperatures for your car, particularly during seasonal travel, mean more use of your air conditioner and an even stronger need to keep your engine well lubricated with enough coolant and oil. All fluid checks should be done while your engine is cold. This is VERY important because hot fluids can burn you.

All fluid checks should be done while your engine is cold. This is VERY important
because hot fluids can burn you.

  • Coolant: To check your coolant, unscrew the radiator cap and make sure the level is where it should be according to the line on the tank. If you’re low, topping off with a pre-mixed 50/50 coolant will save you time and fuss.
  • Motor oil: To check your oil, pull out the dipstick, wipe it down with a rag, and stick it back in. When you pull it out again, the oil should be at or above the line marked on the stick. If it’s too low, add a quart. Your owner’s manual will tell you what kind you need.
  • Windshield washer fluid: While you’re at it, check your windshield washer fluid. Your car may be able to survive without it, but a clear view of the road can be a life saver—literally. In most vehicles, washer fluid is blue and housed in a white plastic tank. Look on the side of the tank or open the cap covering the tank to check the fluid level.

For more helpful tips on checking essential fluids, read this.

Radiator hoses and spare tires

Fluids aren’t the only things that keep your vehicle running smoothly. Include these items in your regular inspections.

  • Radiator hoses: If coolant is the life blood of your engine, then radiator hoses are the arteries. So, while you’re under your hood, check your radiator hoses for leaks or wear. Squeeze the hoses and make sure they’ve got some give to them. If they’re hard and brittle or cracked, they should be replaced. It’s an easy job and far preferable to breaking down on the side of the road when you’re headed out to the lake with the family.
  • Spare tires: As long as you’re taking stock of your vehicle’s essentials, you may as well check your spare tire. First, make sure you have a spare tire in your car. Many vehicle manufacturers these days are eliminating spare tires as a standard feature. If you have a spare tire, make sure it’s properly inflated. Don’t forget to keep a tire iron in your trunk, along with any other equipment needed to change a tire by the side of the road. You can read through your owner’s manual to get a feel for what it takes to do the job. If you don’t have a spare tire on your vehicle, consider carrying a tire repair kit.

Many vehicle manufacturers these days are eliminating spare tires as a standard feature.

Checking the fluids and hoses on your vehicle will save you money and hassle. They’ll also give you something you can’t put a price on: peace of mind. Are there easy hose and fluid checks you perform on your vehicle? Leave us a comment.