Motor Oil: What Do Grades Mean For Your Engine?

You may have seen oil grades such as 10W-30 on a bottle of oil and been curious about what they mean. More importantly, you're probably wondering what kind of lubricant you should be using in your engine. Choosing the wrong product can prevent a vehicle from starting or even severely damage the engine. Luckily, understanding what the grades mean and what you should put in your car or truck is fairly straightforward. If you just want to know which one to put in your engine, use our oil type lookup. A little knowledge can go a long way to keeping your machine running at its best. Here are answers to your top questions about motor oil grades:

What Are the Numbers on an Oil Container?

The numbers indicate the oil grade, which is a measure of its viscosity. A layman can think of viscosity as a liquid’s thickness. Consider the difference between pouring a glass of water versus waiting for the ketchup to slide out of the bottle. Ketchup has a high viscosity whereas water has very low viscosity.

Now imagine trying to put a number on the difference between the thickness of ketchup and water. This was exactly the problem faced by the Society of Automotive Engineers (SAE) with oil viscosity. The SAE’s solution was the measure the rate at which oil traveled through a standardized opening at specific temperatures.

The grades you see on bottles represent the measurements based on flow time. It’s not as simple as saying “higher numbers are better” because some oils perform better at different temperatures than others. The oil you want depends on your vehicle and the environment in which you drive.

What Does the W Mean?

While many drivers think it means weight, the W in the grade actually stands for winter. The first number can be thought of as the oil’s cold-temperature performance. Oil thickens as it becomes colder, making it harder to pump through your engine. Pancake syrup is an excellent visualization here as it’s fairly thick when cold but almost water-like when hot.

The difference in the viscosity is the reason you see multiple numbers on grades, such as 5W-30. The number preceding the W is a measurement of the oil’s flow at 0 degrees Fahrenheit. The lower the first number is, the better the oil flows at below freezing temperatures. So 0W-30 would be a better cold-weather lubricant than 5W-30.

This is important because if oil becomes too thick it becomes almost impossible for your system to pump. A sudden surge of cold, thick lubricant can put a hole in your filter, preventing it from cleaning your oil properly. If no oil is pumped at all, you experience a catastrophic engine failure. Always use the correct winter ratings when driving in a cold environment.

What Does the Number After the Dash Mean?

While engines are always hot when they run, hot weather puts even more stress on your oil. In the cold, having a thin oil prevents your car’s lubricant from becoming too thick. High temperatures thin out your oil, necessitating a higher viscosity. If oil becomes too thin, it doesn’t clean your engine well. The debris in your system causes poor performance and gas mileage. Even worse, thin oil doesn’t absorb enough heat, which can cause your engine to overheat and fail.

Think of the number after the dash as the oil’s high-temperature rating. In this case, the SAE measures the time it takes for the oil to travel through an opening at 212 degrees Fahrenheit. The higher the second number of the grade is, the better the oil stands up to heat. Using 20W-50, for instance, would be an excellent choice if you lived in the tropics.

That’s why 5W-30 is one of the most commonly used lubricants. In temperate regions, such as most of the United States and Europe, it works well whether you’re driving in the summer or winter. This is why multi-grade oils have become so popular in recent years: they provide better overall protection than monograde lubricants.

What Does It Mean When There’s No Dash?

If you see a rating such as SAE 30, this is a monograde oil. Chances are this bottle is not designed as an automotive lubricant. All modern engines use oil whether they’re on cars or not. Some don’t need the advanced lubricants used by today’s automobiles. Riding mowers, generators, and chainsaws can run just fine on monograde oil.

You may be wondering, “Can you use SAE 30 motor oil in a car?” The answer is that this type is generally suitable only for antique cars. Even if it rarely gets cold near you, it’s best not to risk your engine during an early morning ignition. Multi-grade oils protect your vehicle and help it to work better as well.

What Kind of Oil Should I Buy?

You don’t have to worry about the exact temperature of your driving environment every time you buy oil. Your car or truck has all the info you need about purchasing oil in its service manual. It should tell you the exact type and grade used by your engine. It may even have recommendations for cold- or hot-weather oils if you’re doing some traveling.

If you live somewhere with wide temperature swings, don’t fret about finding the perfect lubricant.

5W-40 oil works well in freezing or sweltering conditions, so it’s always a good choice provided it matches your engine. Even if it doesn’t, don’t forget that you change your oil multiple times during the year. This gives you the opportunity to switch grades to match the next season’s weather.

While motor oil grades can be surprisingly complex, finding the right one is fairly easy. As long as you check your owner’s manual you should get the right lubricant. If you’re interested in saving money, do your own oil changes. Not only is it much cheaper, you get a better feel for the condition of your vehicle. Oil replacements are the most important kind of regular maintenance on your car, so it’s a good idea to take charge of them yourself. Shop engine oil online to find high-quality lubricants at the right price.

Learn more about motor oil:

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

FREE Loan-A-Tool® program requires returnable deposit. Please note that the tool that you receive after placing an online order may be in a used but operable condition due to the nature of the Loan-A-Tool® program.