Oil Viscosity Explained: 5w20 vs. 5w30 vs. 10w30

Motor oil’s main purpose in the engine is to lubricate metal parts so that they do not rub together, create friction, and wear down. The lubrication also keeps the engine significantly cooler. In addition, motor oil seals piston and cylinder gaps while coating and protecting engine parts from sludge acid, water, and other elements that cause rust and corrosion. Finally, motor oil cleans acids and silicon oxide from engine parts. An important factor in all these functions of motor oil is the oil viscosity. Read on to have oil viscosity explained in simple terms.

Oil Viscosity Explained

Different Types of Motor Oil

The term “viscosity” refers to the thickness of the oil and its resistance to flow. A higher oil viscosity number indicates a thicker oil. Where this is vitally important is in an engine’s bearing clearances. Older engines were built with much looser tolerances, which allowed oils such as 10W30 to be commonplace as an OE recommended oil in the 1990’s and before. More recent engines have been built with better technology, and much more strict tolerances, which also means a much thinner oil is necessary to properly lube the engine’s bearings, and circulate quicker and freer throughout the engine, and often times with less overall oil in the system. A thinner oil reaches the engine parts quicker, and circulates faster, while a thicker oil tends to adhere more easily to the engine parts, and will provide better lubrication when engine tolerances have deteriorated, or in severe-duty applications like racing or off-road engines. Viscosity will determine how your engine’s oil will deal with changes in temperature, pressure, and speed. What’s vitally important, is that you stick with what the manufacturer recommends for oil viscosity.

What Do Oil Viscosity Numbers Mean?

The five and 10 digits refer to the viscosity at cooler temperatures. So, that means when you are comparing 5w30 vs. 10w30, the thinner 5w30 will circulate quicker, and flow through bearing tolerances better during an engine startup in the winter or colder climates. The “w” behind the five and the 10 indicates that those are the viscosities in colder or winter temperatures. The viscosity of the winter number is tested at 0 degrees Fahrenheit. The 30 refers to the viscosity at higher temperatures. This indicates that the oil is thicker at higher temperatures, so it adheres to the metal parts of the engine, protecting them as they operate. This number is tested at 212 degrees Fahrenheit. The viscosity number is the result of the Society of Automotive Engineers (SAE) determining how fast the oil will flow through a certain size tube.

When comparing 5w20 vs. 5w30 motor oil, the 20 indicates that the oil has a lower viscosity and is thinner at higher temperatures. This enables the 5w20 motor oil to reach engine parts quicker and create less drag that reduces fuel economy. So, due to viscosity, 5w20 is a thinner oil during operating temperatures, whereas 5w30 is thicker during operating temperatures. A majority of manufacturers are now producing engines that operate on 5w20 oil, to which it’s important to stick to this viscosity and not substitute 5w30.

Why Do Some Oils Only Have One Number?

Originally, oils were only produced in a single grade, such as SAE 30. At that time, oils had to be changed seasonally using a higher oil viscosity in the summer and a lower viscosity in the winter. Then, thickening agents were developed and multigrade oil became available.

Which Viscosity Is Best?

Using the motor oil recommended in your owner’s manual is the best way to protect your engine, have it operate efficiently, and get good mileage. However, many owner’s manuals could provide a range of oils. So, since the difference between 5w20, 5w30, and 10w30 is the temperature at which they work best, the answer depends on where you live and what the climate is. If your car’s manual suggests a specific weight, as most cars built in the last 20 years will – stick with it.

Using a Higher Oil Viscosity in High-Mileage Cars

Like mentioned before, in the past, engines were built with looser tolerances – especially bearing clearances. An engine’s life began to deteriorate rather rapidly around 100,000 miles, and so to increase the oil pressure, high-mileage car owners would use a higher oil viscosity such as 10w40 or 20w50. It’s important to note, that your engine’s oil pump, and thus the engine, has to work extremely hard to pump thicker oil and maintain oil pressure, so folks that would resort to 20w50 oil on their tired, old engine would also sacrifice horsepower using it. Fast forward to today’s engines, and it’s not uncommon to see engines on the road with 300-400,000 miles on them. Many circulation and oiling issues of the past have been solved, along with tighter tolerances, so switching to extreme weights of oil is not always necessary, and could actually cause your engine to perform worse. If you notice the motor burning or leaking oil, you can use a high-mileage motor oil rather than a higher viscosity. High-mileage motor oils have additives that restore seals.

Final Words

To recap, a lower oil viscosity number indicates a thinner oil that will flow faster, while higher oil viscosity number indicates a thicker oil which often works better in extreme applications. For engines that do not have a recommended weight – winter and colder temperatures, you should use a lower viscosity oil for cold start up. For warmer temperatures, you should use a higher viscosity oil.

In addition, motor oil not only lubricates engine parts, but it also cleans acids and silicon oxide from engine parts. These waste materials remain in the motor oil until you change your oil. So, it is important that you change your motor oil and oil filter regularly as recommended by your owner’s manual.

For answers to any questions you have about changing your oil, visit your nearby AutoZone store. Our knowledgeable employees can help you select the motor oil, oil filter, and other items you need to change the oil in your car. You can also shop online using our in-store pickup or our next-day delivery service. To find an AutoZone store near you, use our store locator online. After you have changed your oil, bring your used oil to AutoZone for recycling.

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.

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