Understanding Oil Ratings

August 14th, 2020 by

An oil light is shown on an information cluster.

First things first – to be completely honest, you don’t really need to know what the oil ratings on a bottle of motor oil mean. If you have a mechanic who you trust and you never need to personally add oil to your engine, then you don’t really need to know all of this. But whether you have a professional that you like to visit for an oil change near you, or you do it yourself in your garage, it’s still a good idea to understand what the different codes and ratings on motor oil mean.

By having a basic understanding of this stuff, you’ll be able to make sure your mechanic uses the right oil for your vehicle. And if you need to add oil to your engine – even if you don’t have a leak, sometimes you need to top it off – then you’ll be able to read different labels and make sure you pick just the right one. Don’t worry, we’re not going to get into all the specifics and science behind this stuff, but by the end of this brief exploration of motor oil, you’ll know what you need to be confident in caring for your vehicle. Now let’s jump in.

Oil Category vs. Oil Grade

Before we go any further, we need to clarify something. When looking at the “rating” on a bottle of motor oil, there are actually two different things you need to check out and recognize. One is the oil category, and the other is the oil grade (sometimes also referred to as “oil rating”).

The oil category tells you what kind of vehicle the motor oil is meant for, in terms of how old the vehicle is. If you’re driving a modern SUV, you can’t use the same oil that you would have put into a car from the 1960s because it has changed since then. But since classic vehicles are still so popular, there are plenty of legacy oils still available for you to choose from.

On the other hand, the oil grade, or rating, indicates the viscosity of the motor oil. Viscosity is a word that here means “thickness of the fluid” (technically, it’s a bit more complex than that, but you don’t really need to know exactly how it’s categorized). Think of it like the difference between maple syrup and water: syrup has the greater viscosity; it’s thicker than water and will coat the back of a spoon.

A gloved hand is checking the oil dipstick on a car in a garage in Romeoville, IL.

Understanding Oil Category

The category on a bottle of motor oil is indicated as a pair of letters following the words “API SERVICE” on the little circle on the bottle. The American Petroleum Institute (API) has set the standards for categorizing different types of motor oil throughout the years. Fortunately, this isn’t particularly complex (it is if you get into all the details, but we’re not worried about that today), and uses a simple format: the letter “S” followed by a second letter. The “S” has remained since the very first category, while the second letter has increased with different generations of vehicles and oil.

In other words, SA was the first category for motor oil and is appropriate for vehicles built before 1930. You can still find SA oil for sale, but you only want it if you have a very old vehicle – in a modern vehicle, it will do a terrible job and could damage your engine. From there, SB was the category used for vehicles built between 1931 and 1950, then SC for 1951 to 1966, etc. For reasons we won’t get into, the API decided to skip SI and SK.

SL motor oil was designed for vehicles made in 2001, while SM is for 2004 vehicles, and SN is for those made from 2011 and newer. The most recent generation of motor oil is SN, though SN PLUS is available and currently in use as well. To make sure you know exactly which category of motor oil is right for your vehicle, look in your car’s owner’s manual, where it will clearly indicate the type of oil to use.

Understanding Oil Grade

Oil is being poured into an engine during an oil change near you.

Oil grade is not based on when your vehicle’s engine was made and instead refers to the viscosity of the oil itself. As we said earlier, this essentially means how thick the motor oil is. You want your oil to be thick enough to coat and lubricate the moving parts in your engine properly, but not so thick that it can’t easily flow through the engine.

Modern motor oil has a grade indicated by one or two numbers, the letter “W,” then two more numbers. Some people believe the “W” stands for “Weight,” which is incorrect – it actually refers to “Winter.” That’s because the first numbers, before the W, tell you the viscosity in extreme cold, while the second numbers tell you the viscosity in extreme heat.

Exactly what the numbers mean is a matter of measurement by the Society of Automotive Engineers (SAE) that established this grading system; all you need to know is that higher numbers mean greater viscosity. The first numbers indicate the viscosity when measured at 0 degrees Fahrenheit, while the second numbers indicate the viscosity at 212 degrees Fahrenheit.

So, if you see an oil that says “5W-30,” then that means the oil won’t be too thick in cold temperatures. By comparison, a “10W-30” oil is thicker in cold temperatures but still has the same viscosity in higher temperatures. Again, your vehicle’s owner manual will tell you what grades of motor oil are best for your car, though it might indicate more than one.

Which Oil is Right for Your Vehicle?

As we said, you’ll want to choose the category of motor oil that is right for when your vehicle was made – or at least the engine. If you have a classic car chassis with a brand-new engine in it, then you’ll need to buy a more modern motor oil for your vehicle. So it’s important to know when the engine was built to ensure you choose the right oil.

In terms of grade, you can use your car’s owner’s manual to guide you, but your location will also impact which oil you choose. Here in Romeoville, we don’t face the kind of harsh winters that people in Michigan do, so a 10W motor oil is typically sufficient. We also don’t face extreme heat beyond what your motor oil will deal with inside the engine during normal use. This means that your average 5W-30 or 10W-30 oil is likely to work just fine. Remember, before you choose any motor oil, be sure to check your owner’s manual to see what it recommends.

Is it Time for an Oil Change?

Now that you have a better sense of what all that information on a bottle of motor oil means, you’re ready to choose the right oil for your vehicle. If you don’t want to change your own oil, or you still have more questions, then we can help. Come visit us at Romeoville Toyota today to talk with one of our expert mechanics, and we’ll make sure your vehicle is well cared for.

Posted in Oil Change