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Understanding Oil Viscosity: What Do Those Numbers On The Bottle Mean?

Understanding motor oil viscosity

Oil viscosity can feel a bit like math class when you’re standing inside your local auto parts store staring down row after row of oil bottles plastered with algebraic numbers and letters. To make it simple, just pick up the oil type that’s recommended in your owner’s manual. In fact, you should ALWAYS follow the vehicle owner’s manual to determine the correct viscosity grade, engine oil specification, and oil drain interval.

All About FlowThermometers

In the simplest terms, viscosity refers to how well a lubricant flows at a given temperature. The quicker an oil flows, the lower its viscosity and the grade assigned to it by the Society of Automotive Engineers (SAE). High viscosity oils flow more slowly and come with a higher SAE number. It’s logical to assume that a low viscosity oil is thinner than a high viscosity oil. Sometimes, viscosity is also referred to as an oil’s “weight,” with a heavier oil featuring a higher viscosity grade.

How Is Flow Measured?

It wouldn’t make much sense to rate engine oil flow at room temperature — after all, it gets really, really hot inside your engine, and that’s where it’s crucial that oil flows as efficiently as possible for a given engine’s design requirements. The SAE defines high temperature oil viscosity at 212 degrees Fahrenheit (which is 100 degrees Celsius), which is the baseline used to approximate the operating temperatures of most vehicles. It is the operating temperature viscosity that is most important. A viscosity “grade” is just like a grade in school that denotes a range of numbers. For example, an A is a 90-100, a B is an 80-89, etc. Viscosity grades are similar. When you see an oil that’s rated SAE 30, this translates into an engine oil that has a viscosity between 9.3 cSt and 12.5 cSt at 212 degrees F.

Multigrade Oils

Sometimes it gets much colder than room temperature — particularly during the winter months when the thermometer can plunge well below freezing. It’s crucial that oil flows just as well when you first crank the ignition as it does when the vehicle is warmed up, otherwise there’s a risk of damaging internal components.

This is why multigrade oil viscosity is the standard in the industry. You might have noticed that there are far more bottles of oil with a “W” on the label than there are with just a single SAE number. That W stands for “winter” and is a shorthand that tells you that you’re dealing with a multigrade oil that’s been tested to pump at extremely cold temperatures. The number preceding the W indicates the lubricant’s viscosity grade when operating at extremely cold temperatures. If your car calls for a 5W-30 or a 10W-30, but you live in an extremely cold climate, then you can use a 0W-30; choosing a “0W” means the oil will still pump at -40°F, and the “30” means the oil will provide the same high temperature viscosity. Lower viscosity “0W” oils can also help provide better fuel economy, because it is easier for an engine to pump a thinner oil. And remember  that viscosity measurement rules are the same whether you choose a normal conventional motor oil, or a high tech natural gas derived full synthetic motor oil like Pennzoil Platinum.

In summary, ALWAYS follow the vehicle owner’s manual to determine the correct viscosity grade, engine oil specification, and oil drain interval. However, if you live in an extremely cold climate or just want better fuel economy, you can use a “0W” viscosity grade while keeping the recommended operating temperature viscosity grade (the second number) the same (e.g. SAE 0W-20 is compatible with a SAE 5W-20).

Check out all the chemical products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on oil viscosity, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Pennzoil.

Benjamin Hunting View All

Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time.  I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

One thought on “Understanding Oil Viscosity: What Do Those Numbers On The Bottle Mean? Leave a comment

  1. Good report on oil, oil use and the oil ratings. I am better informed to take care of my engines. Thank you.

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