Motor Oil: What Do the Numbers Really Mean?

Our Mechanic Next door delves into the origins and meaning of motor oil viscosity grades.

“220. 221. Whatever it takes.”

motor oil 1

That infamous line of reasoning worked for Jack Butler (Michael Keaton) in the 1983 movie Mr. Mom, so 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.

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.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.

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.

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.

How a car engine works – an animated tale

Check out this amazing infographic on car engines by artist and car enthusiast Jacob O’Neal.


When asked about his inspiration, Jacob gave us the following background info:

“I believe car engines are works of art, and thus, [make] the perfect subject matter. I designed this graphic to satisfy a deep need to produce something amazing and totally different from what’s currently out there in image-based infographics.”

We appreciate Jacob’s sense of creativity and keen knowledge on how car engines work. Feel free to share this page with friends!



Fluids, hoses and a little more peace of mind

hood up fixing carHey friends, this time out, we’re gonna delve into the so-called brains of the operation—the car engine. Well, not the engine itself per se, but some of the things that will contribute to its sustained health.

But don’t worry and crinkle your face like my wife does when I try to smoke a cigar inside. What I’m gonna suggest today is just common-sense maintenance, and it’s mostly based around checking your fluids and hoses.

Fluids and Hoses

As you know, your car has a lot of fluids running through it. As your body needs water, your car needs engine oil and coolant, among other things, to keep its engine running smoothly and not too hot. So it’s always a good idea to make sure those fluids are not only at the right levels—and not leaking.

High operating temperatures for your car, particularly during seasonal travel, means more use of your air conditioner and an even stronger need to keep your engine well lubricated.

And as with tire pressure, all checks should be done while your car is cold. This is VERY important because hot fluids can burn you.

First of all, make sure they’re topped off. With coolant it, just means unscrewing the top and making sure the level is where it should be according to a line on the tank, and with oil it means pulling out the dipstick, wiping it down with a rag and sticking it back in. When you pull it out, it 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. This is a very simple bit of basic car maintenance and it’s a good idea to check every time you fill your tank.

Also look under your hood and check for leaks. Squeeze the rubber radiator hoses and make sure they’ve got some give to them. If they’re hard and brittle, or worse yet, cracked, they should be replaced. It’s an easy job and it’s far preferable to breaking down on the side of the road when you’re headed out to the lake with the family.

What Do You Carry?

Make sure you have plenty of windshield wiper fluid. Even if it’s not raining, you need to be able to see clearly. Plus, when bugs get stuck to your windshield, your spouse may get antsy…my wife does.

Also make sure that you have a spare tire in your car that’s properly inflated, and you’ve got all the equipment needed should you need to change a tire by the side of the road. Read through the section of your owner’s manual to make sure you have all the necessary equipment and get a feel for what it takes to do it. It’s also a good idea to keep some basics in your car for emergencies: a blanket, a bottle of water, an extra bottle of radiator fluid and windshield wiper fluid.

As I like to tell my friends, get to know your car a little. Become a little familiar with the engine bay and check things out and it will not only save you money and hassle, but will give you something you just can’t put a price on: peace of mind.

Editor’s note: Advance Auto Parts carries a wide selection of hoses, fluids and other quality auto parts. Get back to the garage fast—buy online, pick up in-store…in 30 minutes.