You know it’s time to top up your car’s oil, but as you stare at the rows and rows of labels in the motor oil aisle, you wonder, “what do all these numbers on the oil mean?” While names like 5W-20 and 10W-40 might seem like secret code, the meaning behind the numbers and letters on engine oil containers is actually pretty simple. Each combination represents the oil weight of the motor oil inside, which lets you know if the oil you’re choosing has the correct viscosity for your engine.
Learn what oil weight means, what viscosity is, how to read oil weight on labels, and the different car oil types you can choose from, so you can make an informed decision about what goes into your car.
Engine Oil Grades Explained
Engine oil grades—also known as oil weight or viscosity grade—measure how motor oil flows in an engine. Knowing your vehicle’s recommended oil weight is important because engine components rely on specific grades of oil to run smoothly and stay cool at operating temperatures upwards of 200 degrees Fahrenheit.
But not all engines are the same—a four-door sedan, motorcycle, and semi-truck all need different types of oil. That’s why the Society of Automotive Engineers (SAE) developed the naming conventions we use today.
What Does SAE Mean on Oil?
SAE, the organization responsible for standardizing how we refer to auto oil viscosity specifications, designates 14 different oil grades on 2 scales, from low to high viscosity: 0W, 5W, 10W, 15W, 20W, 25W, and 8, 12, 16, 20, 30, 40, 50, and 60.
You might see some of these numbers on their own, indicating a single-grade oil like SAE 10W, and some numbers in combination, indicating a multigrade oil such as SAE 10W-20. Either way, if you see the letters “SAE” preceding the oil weight on a jug of motor oil, you know that oil meets the industry-defined viscosity.
What Do Engine Oil Numbers Mean?
Here’s the quick answer:
- The First Number (before the W): This number represents the oil’s cold-temperature viscosity. The lower the number, the better the oil performs in cold weather. For example, 5W oil flows better at low temperatures than 10W oil.
- The "W": This stands for “winter,” indicating that the oil is tested for performance in cold temperatures.
- The Second Number (after the W): This is the oil’s high-temperature viscosity. The higher the number, the thicker the oil remains when your engine gets hot. So, 30-weight oil (like 5W-30) will flow better at high temperatures than 40-weight oil (like 10W-40).
When it’s time for your next oil change, bring your car into Tires Plus for fast and friendly service. We’ll help you determine the best oil grade for your engine, and answer any questions you have about what engine oil numbers mean so you can make an informed decision for your vehicle in the future.
Engine Oil Viscosity 101
To fully understand engine oil numbers, you need to understand viscosity. Put simply, viscosity is a liquid’s resistance to flow. In other words, how thick it is. Engine oils with high viscosity, like 15W-40, are thicker and flow more slowly than low viscosity oils like 0W-20, which flow more easily.
Temperature also has a big impact on viscosity. Oil becomes thinner at higher temperatures and thicker at lower temperatures. If you’re not using the correct viscosity for your engine, the oil may thin out too much to effectively lubricate engine components or be too thick to flow throughout the engine.
While all oils get thinner and thicker with temperature changes, a heavier-weighted oil like 25W will still be thicker than a 10W oil at the same temperature. The amount that a motor oil’s viscosity changes across temperatures is known as its viscosity index.
How to Read Oil Weight
Let’s put it all together using 5W-30, one of the most commonly used oil weights.
- 5: This number represents the oil’s viscosity rating when cold. “5” is the second-lowest cold oil weight, only above a 0 viscosity rating. This means that when the engine is cold, the oil still flows very easily.
- W (Winter): A winter rating tells us that, in addition to a cold engine, this oil is also rated to flow easily even when it’s cold outside.
- 30: This number represents the oil’s viscosity rating for high temperatures—specifically, how the oil performs at 100 degrees Celsius (212ºF) and above, which is about the same temperature as a fully hot engine.
It’s important to note that the numbers 5 and 30 don’t refer directly to temperature, but rather a numerical rating that tells us how the oil will perform as the engine’s temperature moves from cold to hot.
Car Oil Types Explained
While oil weight describes how well motor oil flows at different temperatures, a car’s oil type refers to its actual composition of its largest ingredient. There are primarily two types of engine oil: conventional and synthetic.
For a full breakdown, check out our article on the good, better, and best oil types for your vehicle.
Conventional Oil
Conventional oil is derived from crude oil and, while it contains some impurities, it does a good job of lubricating most engines to keep them running smoothly. It’s a dependable, affordable option for everyday driving conditions and older vehicles you don’t intend to drive much longer. However, it tends to break down sooner under high heat and heavy loads. This means conventional oil may not offer the same long-term protection you get with synthetic oil.
Synthetic Oil and Synthetic Blends
Synthetic oils and synthetic blends are typically designed to offer more advanced protection and performance compared to conventional oils. These oils are typically more refined to flow better in cold temperatures and resist breakdown under high heat.
Synthetic blends bridge the gap between cost-effective conventional oil and high-performance synthetic oil. They use conventional oil as a base, but usually add synthetic components to enhance performance and durability.
Fully formulated full synthetic oil usually delivers the best protection to help you get the most out of your engine. It maintains its viscosity better over time, reducing engine wear and improving efficiency. For drivers who put their vehicles through more demanding conditions, full synthetic is often the best investment in long-term performance.
How to Choose the Right Oil Type and Weight
When choosing the best motor oil for your car, consider these three factors: vehicle age, driving conditions, and manufacturer specifications.
Manufacturer Specification
The first and most important consideration is your vehicle manufacturer’s specification. Car manufacturers build and test engines with very specific oil requirements in mind. So whether it’s SAE 5W-20 or SAE 15W-40, conventional or synthetic, the right motor oil for your car is usually the one listed in your owner’s manual.
Note: In addition to the SAE viscosity grade, vehicle manufacturers also specify the minimum performance specifications the oil must meet. Examples include API SP, ILSAC GF5, dexos, etc.
Vehicle Age
Next, consider your vehicle’s age. Older engines over 75,000 miles may benefit from a high-mileage oil designed to reduce leaks, oil consumption, and engine wear.
Driving Conditions
Finally, consider your average driving conditions. If your engine is regularly strained by frequent short trips, extreme hot or cold temperatures, heavy loads, or heavy traffic, you’ll want the protection and durability of full synthetic oil. For easy driving and everyday commutes, a more affordable conventional oil may work just fine.
Unsure? Get a Second Opinion
Talk to the pros at Tires Plus. Choosing the wrong oil for your car can cause undue strain on the engine, leading to reduced fuel efficiency, poor performance, and even lasting engine damage. When in doubt, our professional technicians will help you understand all your options based on your specific make, model, vehicle age, and more, so you can give your engine exactly what it needs.
Frequently Asked Questions
Is Synthetic Oil Better than Conventional Oil for All Engines?
While it’s true that synthetic oil generally offers better protection and performance, especially under demanding driving conditions and extreme temperatures, it’s best to follow your vehicle manufacturer's specificationss on which type of oil to put in your engine.
Which Engine Oil Grade is Best?
There is no “best” engine oil grade—just the one that’s best for your specific car based on your vehicle's specifications and driving conditions. Refer to your owner's manual or consult with a Tires Plus technician for guidance.
Can Using the Wrong Car Engine Oil Grades Damage an Engine?
Yes. Using the incorrect oil grade can lead to inadequate lubrication, increased friction, and potential engine damage due to the oil being too thick or thin to flow properly through your engine.
What Do the Numbers on Motor Oil Mean?
The numbers on motor oil refer to the oil’s viscosity grade, where the first number represents the oil’s viscosity when cold and the second number represents the oil’s viscosity at normal engine operating temperatures. A “W” between the numbers, like in “5W-30,” indicates the oil is rated for cold-weather use. The “W” stands for “winter.”
How Does Oil Viscosity Affect Engine Performance?
Oil viscosity affects how easily oil flows through the engine, a crucial factor in engine performance. Proper viscosity ensures there’s enough lubrication, reducing bearing wear and improving fuel efficiency.
Which Oil is Thicker: 5W-20 or 5W-30?
While both oils have the same “5W” rating for cold-weather performance, the 5W-30 is a bit thicker at higher temperatures. That means it provides slightly more protection under heat and heavy loads.
Which Oil is Thicker: 5W-30 or 10W-30?
At operating temperature, these two oils flow at about the same thickness, as indicated by the “30” viscosity rating. At lower temperatures, however, the 5W-30 flows more easily than 10W-30, making it a better choice for cold starts in chillier weather.
Which Oil is Thicker: 10W-30 or 15W-40?
15W-40 is the thicker oil, both for cold starts and when the engine is hot. This oil weight is often used to protect engines in diesel trucks and other heavy-duty vehicles.
Get the Right Oil for Your Ride
Confused about oil grades, weights, or what your engine really needs? The pros at Tires Plus can help decode the label and keep your engine running strong with the right oil for your ride. Schedule your next oil change online or stop by your nearest Tires Plus for fast, professional service you can trust.
