What Happens if I Use the Wrong Weight (Viscosity) of Oil?

amsoil engine oil weight

It depends on how your engine is built and how it reacts to the specific viscosity of oil you use.

That’s pretty nebulous, so here are some examples.

Josh buys a new car that requires 0W-20 motor oil. He hangs around enough gearheads to have heard the old axiom that “higher viscosity oil equals better wear protection.”

Wanting the best protection possible for his new ride, he drains the 0W-20 that came from the factory and uses it as bar and chain oil in his chainsaw. Then he installs 15W-50 racing oil.

Next we have John. He’s a cost-savings enthusiast, so he buys a 1998 Toyota Corolla with cash. It calls for 5W-30 motor oil. But he has some 10W-30 in his garage, so he uses it to change the oil. No sense wasting good oil.

What’s going to happen?

Engines are built to use a certain viscosity of motor oil. Today’s advanced engines, for example, are built with much tighter tolerances than their predecessors. The clearances between the crankshaft journals and main bearings are tighter, for one. This is purposely done to allow modern engines to use lower-viscosity motor oil, like 0W-20 – and even 0W-16, which is used in the 2018 Toyota Camry.

Why? Because lower-viscosity oils reduce internal friction since they flow more easily than higher-viscosity oils, improving fuel economy. With fuel-economy standards growing more strict, automakers are leaning toward low-viscosity lubricants to help them meet the requirements.

In Josh’s case, his 15W-50 racing oil may be too thick to flow quickly enough to fill the spaces between the crank journals and main bearings while the engine is running. The oil won’t form a consistent lubricating film, allowing metal-to-metal contact and wear. His engine was designed specifically to use a lower-viscosity oil, in this case 0W-20. Its lower viscosity allows it to flow faster and fill the tiny clearances between parts, leading to a durable, consistent lubricating film

Not only that, but the engine will waste energy pumping the thicker motor oil, reducing fuel economy. Since thicker oils don’t transfer heat as well as thinner oils, operating temperatures will increase, too, possibly leading to accelerated chemical break down and harmful sludge and deposits.

John is much smarter

In John’s case, the difference between using 10W-30 and 5W-30 is less pronounced. His older engine isn’t built with the same tight tolerances as Josh’s engine. Also, both oils are the same viscosity once the engine has reached operating temperature. He knows this because the second number in each oil’s viscosity rating (i.e. “30”) is the same. It describes the oil’s resistance to flow at 212ºF, or normal operating temperature.

Using 10W-30 instead of 5W-30, however, could make cold starts more difficult, however. It’s helpful to think of the “W” as standing for “winter.” The lower the oil’s “W” viscosity, the more readily it will flow when cold. In this case, 5W-30 will flow more easily at startup than 10W-30.

Thin may not be in

What if John went off the deep end and used 0W-16 in his 1998 Corolla instead of the recommended viscosity?

Just like using an oil with too high a viscosity may lead to problems in Josh’s engine, using an oil with too low a viscosity can have the same result. Oil that’s too thin can fail to develop a consistent lubricating film, inviting metal-to-metal contact that causes wear.

Extreme stress and heat add to the challenge. Since oil thins as it’s heated, the already-too-thin oil becomes even thinner under extreme heat, worsening the problem.

Oil that’s too thin can also lead to insufficient oil pressure to properly operate your vehicle’s variable valve timing system, if equipped. Low pressure may also result in lifters not staying in contact with cams, causing noise and increased wear.

Bottom line…

If you use a viscosity that’s one grade higher or lower than what’s recommended for your engine, it’s unlikely you’ll do any lasting harm. But, to alleviate any concerns about engine protection and your vehicle warranty, it’s best to use the viscosity recommended in your owner’s manual.

If you have questions or concerns, contact AMSOIL Technical Services ([email protected]) to determine the appropriate viscosity for use.



    1. Hi Zack,

      Every engine is slightly different so there’s no way to make specific predictions. However, the manufacturer specifies 5W-30 for a reason. They designed the engine to operate with that viscosity of oil for best protection and performance. Straying from the recommendation could jeopardize wear protection and potentially lead to problems. Stick with what the manufacturer recommends.



  1. I have an 06 Avalanche. It says 5w 30 but my ex put 10w 30 in it. Is that going to be an issue? Im asking because Im not sure what year the tighter restrictions came into play. Thanks ahead of time.

    1. Hi,

      You should be fine. A 10W-30 performs the same as a 5W-30 once you reach operating temperature. With the 10W-30, you’re sacrificing some cold-flow at start up since it’s a little thicker when cold. However, switch back to 5W-30 when you’re due for an oil change.



  2. Amsoil,
    I just purchased a 2019 Dodge Charger with the 6.4 liter Hemi and I noticed that the factory oil is 0W-40. The engine from the factory runs 203 degrees to 217 degrees and the oil runs at 220 degrees to 230 degrees and these are just daily driving being nice. I want to install a 190 degree thermostat and bring both the coolant and oil temps down. Do I run the 0W-30 oil if I drop the oil temps down to 210?

    1. Hi Will,

      The manual on your car specifies a 0W-40 oil that meets the MS-12633 specification, with no secondary option. As with any vehicle modification, you’re assuming a level of risk when diverging from the OEM viscosity recommendation. OEMs base viscosity recommendations on the clearances they designed in the engine; changing the viscosity of the oil could result in insufficient oil pressure or could starve components of oil. The best practice is to stick with the OEM-recommended viscosity.



  3. Hai

    Which oil should i use? My car run 105,000km already. Which right amsoil should i use? Is it 0w20? because i already use 0w20 on my nissan.

    We live in a hot country like asian

  4. Amsoil, I own a 2007 Toyota tundra 5.7L ,. It has 119k miles. I bought it only a year ago, only really drive to work and back, so far it’s been everything you hope a Toyota would be… Including quiet when running. About a month ago I started getting the dreaded cold start valve tick , Even in relatively mild temps (48-50) it does fade away after a few minutes, but all I can think about is the damage happening every time I start it. The only thing I’ve done differently , starting last August, was add NMF ionic friction eliminator. This stuff works great in my big rig….but I’m wondering if it’s the cause of the tick developing in the Toyota. It says it’s safe for all motors. What can I do to quiet the old girl? Thanks in advance! ,. Joe M. ,. S.C.

    1. Hi Joe,

      Make sure you’re using the correct viscosity for your Tundra, which is 5W-20. A heavier viscosity could cause noise at startup. Also, AMSOIL doesn’t recommend using aftermarket oil additives. They can disrupt the motor oil formulation and lead to problems. Check out this post for details.

      Thanks for reading.

  5. What do you recommend as mileage piles up? I find GM lifters don’t tolerate 5.20 well after 150 and consumption goes up as well.

    1. Hi James,

      Once the vehicle is out of warranty, some people like to stray from the viscosity recommendation in the owner’s manual based on vehicle performance and what you’re trying to accomplish. For recommendations, we need more information from you. It’s best to contact AMSOIL Technical Services ([email protected]). They’ll be happy to help.


  6. Amsoil, I have a question is there a actual shelf life on our oils? I am a dealer in Ohio I come into this question all the time. Thank you, Richard Smith /zo# 229430

    1. Hi Richard,

      Product shelf life varies significantly depending on product design, environmental contamination, chemical contamination and temperature. With proper storage, most liquid lubricants can last five years. Because grease can harden or lose its oil content over time, it generally has a storage
      life of two years. For questions regarding shelf life, call AMSOIL Technical Services at (715) 399-TECH.



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