What Does 'W' Stand For In Engine Oil?

If you've ever performed your own oil change at home or even just popped the hood to check your oil level, you may have noticed the mix of numbers and letters emblazoned on your engine oil fill cap or the bottles of fresh fluid. Common engine oil codes include 10W-30, 5W-30, and 0W-20, but, in reality, there are a huge number of different motor oil classifications and types. While some oils can be used in various different engines, your car's manufacturer specifies a preferred engine oil type. You can find this information by reading your owner's manual, and it's important to stick with the fluids recommended by your vehicle's manufacturer to ensure smooth performance and avoid malfunctions.

But what exactly do those codes mean, and, in particular, what does the "W" stand for in engine oil? When it comes to understanding these codes, the "W" part is perhaps the easiest to remember. It stands for winter, and it alludes to the oil's viscosity or thickness at cool temperatures, like when you first start your engine in the morning. However, understanding that part of the code won't tell you much about the oil. If you want to learn more about what the rest of the letters and numbers mean, including what they mean in conjunction with the "W," stick around. Here's everything you need to know about engine oil codes.

Besides the letter "W," the principal part of engine oil codes also contain a couple of numbers. Usually, there's one number before the "W" and one number following a hyphen after the "W." We've already explained that the "W" stands for winter and indicates how thick or viscous the oil is when it's cold. Well, the number that comes before the "W" explains exactly how viscous the oil is at low temperatures, while the number following the hyphen tells how thick the oil is at hot temperatures.

To make this easier to understand, let's use a popular type of engine oil as an example. You can find 5W-30 engine oil on the shelves of virtually all automotive parts stores, as well as supermarkets like Walmart and even gas stations. It's one of the most widespread types of oil used in modern vehicles, and it's suitable for a wide range of engine sizes and types. 

Based on what we've just learned about engine oil codes, a 5W-30 oil has a viscosity grade of 5 at low temperatures. That means that the oil is thinner when it's colder, which allows it to move through the engine freely to provide lubrication. The second half of the code means that the oil has a viscosity grade of 30 at hot temperatures, meaning that it's thicker when it's warm. That allows the oil to provide lubrication at higher temperatures while protecting internal components against damage and friction.

Besides the letter "W" and between three and four numerical digits, most engine oil containers also feature the letters "SAE." You'll commonly see this initialism in front of the primary oil code — for example, you may see SAE 5W-30 oil or SAE 0W-20 oil. Like the codes themselves, the letters "SAE" have an important meaning, but what exactly is it?

The initialism "SAE" stands for the Society of Automotive Engineers. The organization has been around since the early 20th century and was first formed by a couple of automotive magazine publishers and a handful of volunteers to oversee the burgeoning auto industry. Since then, the SAE has grown into an international presence and counts more than 128,000 automotive engineers and other experts as members.

Today, the SAE is dedicated to sharing automotive knowledge and overseeing industry standards. The organization is involved not only in creating educational material and standards for the passenger car industry — it also works with aerospace engineers and various other technical experts to oversee a wide range of industries. When it comes to engine oil, the SAE maintains a set of oil viscosity standards. These standards allow us to purchase and use engine oil confidently, knowing that it will provide the right lubrication for our engines and match the viscosity requirements set forth by the vehicle manufacturer.