Oil Viscosity Explained
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?
Oil viscosity refers to how thick or thin the oil is and how well it flows at different temperatures. This is represented by the numbers you see on oil bottles, such as 5W30 or 10W30, and it’s measured by the Society of Automotive Engineers (SAE). The first number, such as the “5” in 5W30, indicates how the oil flows in cold temperatures (the “W” stands for winter). A lower number means thinner oil, which circulates faster during cold starts, making 5W30 better for cold climates compared to 10W30, which is thicker and slower to flow in the cold.
The second number, like the “30” in both 5W30 and 10W30, represents the oil’s viscosity at high temperatures (tested at 212 F). At this temperature, thicker oil, like 30-weight oil, clings to and protects engine parts better when the engine is hot and operating under stress.
In practical terms, oil viscosity affects engine start-up by determining how quickly oil reaches vital parts in colder conditions. Thinner oil, like 5W20, flows faster, reducing friction and wear during start-up and also improving fuel efficiency by reducing engine drag. However, thicker oil, such as 5W30, provides better protection at higher temperatures by maintaining a stronger film over engine parts, crucial in hotter climates or under heavy engine loads.
| Oil Type | Cold Temperature Viscosity | High Temperature Viscosity | Ideal Range | Typical Applications | Benefits |
| 5W20 | Thinner at cold temperatures (flows quickly at start-up) | Thinner at high temperatures | Best for cold to moderate climates | Modern vehicles requiring better fuel efficiency | Better fuel economy, faster flow at start-up |
| 5W30 | Thinner at cold temperatures (similar to 5W20) | Thicker at high temperatures | Suitable for cold to hot climates | Versatile for most engines, including SUVs and trucks | Better protection under higher heat and load |
| 10W30 | Thicker at cold temperatures (slower flow at start-up) | Thicker at high temperatures | Best for warm or hot climates | Older vehicles or engines under heavy load | Better high-temp protection, less evaporation |
Understanding these numbers helps ensure you’re using the right oil for your climate and driving conditions, ultimately protecting your engine and optimizing performance.
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.
Selecting the Right Oil for Your Vehicle
Choosing the right motor oil viscosity is essential for protecting your engine, optimizing performance, and ensuring good fuel efficiency. Your vehicle’s owner’s manual will typically recommend a specific oil or a range of viscosities. Here are practical tips to help you make the best decision:
- Always start with the oil viscosity recommended in your owner’s manual. Manufacturers design engines to run with specific oil thicknesses to maximize performance and protection. Using the wrong viscosity could lead to poor lubrication, increased wear, or even engine damage.
- If your owner’s manual provides a range of oils (e.g., 5W20 or 5W30), consider the climate where you live.
- Opt for thinner oils like 5W20, which flow better during cold starts and help your engine warm up faster.
- Choose 5W30 or 10W30 if you drive in warmer areas. These oils offer better protection at higher temperatures and are more suited for sustained engine loads.
- Driving conditions matter. For city driving with frequent stops and starts, a thinner oil like 5W20 may improve fuel efficiency. For towing, heavy loads, or off-road driving, consider a thicker oil like 5W30 or 10W30 to provide better protection at high engine temperatures.
- Vehicle type is important. Modern cars and smaller vehicles often benefit from thinner oils like 5W20 for better fuel efficiency. Trucks, SUVs, and vehicles under higher stress or load may perform better with 5W30 or 10W30, which offer extra protection in demanding conditions.
By following these tips and considering your climate and driving habits, you’ll choose the oil that keeps your engine running smoothly.
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.
Common Misconceptions About Oil Viscosity
When it comes to oil viscosity, there are several misconceptions that can lead to improper maintenance decisions. Here are some of the most common ones:
1. Thicker Oil Always Provides Better Protection
While it may seem logical that thicker oil offers better protection by creating a stronger barrier between moving parts, this isn’t always the case. Thicker oils can actually increase resistance, making it harder for the oil to flow, especially in cold weather. This can lead to poor lubrication at start-up, increased engine wear, and reduced fuel efficiency. It’s crucial to use the oil viscosity recommended for your vehicle, as it’s designed to balance protection with performance.
2. Higher Viscosity Oils Are Always Better for High-Mileage Engines
Another common misconception is that switching to a higher viscosity oil is always the best choice for high-mileage engines. While thicker oils can sometimes help with minor leaks or oil consumption in older engines, they aren’t a one-size-fits-all solution. Using oil that’s too thick can strain the engine, reduce efficiency, and cause issues during cold starts. It’s better to use a high-mileage oil specifically formulated to condition seals and prevent leaks without drastically increasing viscosity.
3. Any Viscosity Oil Will Work in Any Climate
Some drivers believe that oil viscosity is universal, but oil performance depends heavily on temperature. Thicker oils can struggle to circulate in cold climates, leading to poor lubrication during start-up. Conversely, thinner oils can break down more easily in extreme heat. Always adjust your oil choice based on both your manufacturer’s recommendation and the climate you’re driving in.
Sticking to the correct oil viscosity for your vehicle ensures better performance and protection, regardless of the engine’s age or mileage.
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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.
FAQs
What is the difference between 5w20, 5w30, and 10w30 motor oils?
The difference between 5W20, 5W30, and 10W30 motor oils is how they perform at cold and hot temperatures, with 5W20 being thinner and better for cold starts, while 5W30 and 10W30 offer more protection at higher temperatures.
How do I choose the right oil viscosity for my car?
Follow your vehicle’s owner’s manual recommendations and consider the climate and driving conditions you typically face.
Can I use a different oil viscosity than what is recommended?
It’s not advisable to use a different oil viscosity than what’s recommended. It may impact engine performance and protection.
What are the benefits of using a multi-viscosity oil like 5w30?
Using a multi-viscosity oil provides better cold-start flow while offering stronger protection at higher temperatures.
How often should I change my motor oil?
Motor oil should typically be changed every 5,000 to 7,500 miles, when the oil life monitor indicates it’s time, or as specified in your vehicle’s owner’s manual.
