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

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 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 if I use the wrong viscosity (weight) of motor oil?

Engines are built to use a certain viscosity of motor oil

Today’s advanced engines 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.


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.

Find the Right Viscosity Oil for Your Vehicle

Oil that’s too thick may not flow quickly enough

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.

For a deeper dive on oil viscosity, check out this post.

Some oil viscosity differences are less pronounced

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. 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.

In fact, some automakers allow you to switch to a lower viscosity oil depending on weather.

Find the Right Viscosity Oil for Your Vehicle

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.

Find the Right Viscosity Oil for Your Vehicle


  1. One thing I don’t see clearly answered is a lot of people talking about older cars and the clearances inside the engine increasing over 10-20 years. You focused mainly on older cars not having as tight of tolerances when they are manufactured but I am more curious about the characteristics of engines as the age.

    I know with my 2002 Toyota Corolla being 18 years old the last year or so I have increased the viscosity of the oil I used to reduce the amount of oil it was consuming and just using higher milage oil didn’t stop it as much as using high mileage oil that had a higher viscosity.

    So my question is for 2002 Honda Civic that is rated for 5W-20, I feel like in places where it doesn’t drop below freezing 10W-30 should be recommended over 5W-20 or 5W-30 and in the north in winter 5W-30 should be sufficient still. What are your thoughts on this and is there any science or testing or manufacturer recommendations for cars way outside of their warranty?

    1. Hi Danny,

      Good questions. At the risk of sounding condescendingly obvious, every engine is different. They differ in how they’re made, maintained and driven, so it’s impossible to make a sweeping generalization (i.e. “Use a viscosity grade one number higher in all engines 20 years old or older”) that applies universally in all instances. There simply are too many variables.

      Some manufacturers allow you to use a higher-viscosity oil when driving in warmer weather. Some recommend a single viscosity all the time, so start by checking the owner’s manual.

      If your older engine burns some oil, using a higher-viscosity oil, as you suggest, can help seal the rings and fill those wider clearances. It’s a common practice that motorists have done for ages.

      Our official recommendation at AMSOIL is to stick with the viscosity the manufacturer recommends whenever possible. In older, out-of-warranty engines, however, using a viscosity one rating higher or lower isn’t likely going to cause any issues and can, in cases such as yours, help.

      Personally, in my 1998 Corolla, I use 0W-30, despite it calling for 5W-30. The car doesn’t burn oil, so I see no need to bump to XW-40. And, since I live in northern Minnesota, I like the added cold-flow with the 0W winter rating.

      I hope that helps. Thanks for reading.


  2. 17 Camaro SS calls for 5w30 but ill be running 10w30 Zrod. car doesnt get driven below 40 degrees.

  3. In 2004 I purchased a Dodge Ram with 4.7L 5W30 was recommended, I used it for a while but decided to try 0W30, I seemed to get better fuel economy about 60 to 70 miles more per tank of gas. Is this a good practice? I saw no issues with the truck for 15 years of daily driver service.

    I have recently purchased a 2018 Ford F150 5.0L which 5W20 is recommended. Could using 0W20 instead of recommended 5W20 cause any detrimental issues? I’m expecting possible better performance as previously experienced.

    1. Hi Dan,

      Generally, there are no issues using a 0W oil in place of a 5W, provided it meets the correct specifications. For the 2004 Ram, there are no issues using a 0w-30.

      The 2018 Ford F150 with the 5.0L requires an oil meeting the Ford WSS-M2C945-A specification. None of our 0W-20 oils meet that Ford specification, so we don’t recommend it.



    2. i can tell u f/experience if u use the “wrong” viscosity & car iz under warranty, they WILL reject ur claim… it does not matter if it iz a difference of 5w / 10w & it iz july… it will not matter 2 them…

  4. I build street rods and just finished a 1939 Chevy coupe, installed a new LS3 crate engine and 4L60E transmission. The paper work with it says 5-30W, but says that 10-30W can be okay also. I know the drill on oil weights and your right about colder weather start ups, also a fuel consumption gain with the lighter weight oil. As street rods are warm weather driven vehicles there isn’t a issue with cold starts!

  5. I put 5w- 30 in a car that unexpectedly takes 10W-40 oil. motor’s a base model early 90s motor with high mileage. I assumed it was the proper type cause previous owner seems to have been putting it in (spare 5/30 qts in trunk) . It started and sounds fine but Do you think i should sacrifice this oil and change it again? (It is coming into summer…)
    [1991 Toyota Celica ST]

    1. Hi Skrrt,

      Check the owner’s manual first. Maybe it allows for 5W-30, but if not, go ahead and change to 10W-40.



    2. You might mind an additive that will increase the viscosity instead of changing it again.

    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.



  6. 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.



  7. 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.



    2. warning dodge thermostat is picking up the coolant as it comes out of the hot engine so they use 203 stats on many jap cars the thermostat is picking up the cooled flued thats comming out of the rariator so they have to use 180 themostat so my jeep wrangler with a 203. runs cooler than my toyata that had the 180 stat,

    3. i do believe temp drop will effect pollution controls… they raised temp 2 burn more hydrocarbons … mite effect cat/converter …

  8. 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

    1. I know the corvette calls for 5w30, but says use 15w50 for racing applications. Secondly a car driven in cold weather will have a higher viscosity than one driven in hot weather, again the viscosity range will be pretty wide. It make take 30 minutes or longer for an engine to Come up to temperature in cold weather, heck in extreme cold UT may never reach 180-200°f. Therefore the viscosity of the oil will be substantially thicker as compared to the recommended viscosity.

  9. 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.

  10. 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.


  11. 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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