Can I mix 10W40 with 5w20?
In most passenger vehicles, you should not mix 10W-40 with 5W-20. Use a single oil grade that matches the manufacturer’s specification.
The question hinges on how viscosity grades behave across temperature ranges and how additive packages inside oils interact. Mixing two different grades can produce a blend whose viscosity and protective properties fall outside what your engine requires, potentially increasing wear, reducing efficiency, and complicating warranty coverage.
Understanding what the numbers mean
SAE viscosity grades describe how oil flows at different temperatures. The first number with the W (for winter) indicates cold-start viscosity—the lower the number, the thinner the oil at cold temperatures. The second number indicates high-temperature viscosity—the higher the number, the thicker the oil remains when the engine is hot. Oils labeled 5W-20 and 10W-40 are designed for different operating conditions and come with specific additive packages tailored to those conditions. Mixing them creates an unpredictable blend that may not meet the engine’s required specifications.
Is it safe to mix 5W-20 and 10W-40?
Generally, no. Mixing two different viscosity grades is not recommended because the resulting blend may not meet your vehicle’s specification for cold-start protection, hot-running film strength, or additive performance. This can lead to higher wear during startup, inadequate lubrication at operating temperature, and potential issues with emissions and warranty coverage. If your manual specifies a single grade, that is the grade you should use. If you must top up in an emergency, aim for the closest compatible grade and plan a full, proper change as soon as possible.
Before proceeding with any top-up, consider the consequences and how soon you can restore the recommended specification. The following list highlights the main concerns with mixing these two grades.
- The blend’s cold-start viscosity may be outside the engine’s required range, which can affect lubrication during startup.
- The hot-temperature viscosity could fall short of the engine’s needs, reducing film strength under load.
- Additive-package compatibility may be compromised, potentially increasing sludge risk or reducing detergency and protection.
- The overall performance, including fuel economy and emissions compliance, may deteriorate, and warranty terms could be affected.
Conclusion: Avoid mixing 10W-40 with 5W-20 for normal operation. If you accidentally mix, plan a proper oil change as soon as you can and avoid running the engine long-term on the mixed blend.
Emergency steps if you must top up now
If you’re temporarily unable to obtain the exact grade your owner’s manual calls for, take steps to minimize risk and document what you did until you can return to the correct specification.
Before a list of actionable steps, note that these are situational and not a substitute for a proper oil change with the recommended grade.
- Top up with the grade closest to the specification listed in your manual, not a widely different grade.
- Add only enough oil to bring the level to the correct mark on the dipstick—do not overfill.
- Avoid repeatedly mixing different grades; treat this as a temporary measure.
- Schedule a service as soon as practical to perform a full oil change with the correct viscosity and the appropriate additive package.
Following these steps helps limit potential short-term risk, but the preferred approach remains using the exact grade recommended by the manufacturer.
Summary: For engine health and warranty reasons, do not routinely mix 10W-40 with 5W-20. Use a single oil grade aligned with the vehicle’s manual, and if you must top up in an emergency, choose the closest compatible grade and arrange a proper oil change promptly.
What oil can I substitute for 5w20?
5w20 and 5w30 is interchangeable, meaning you can use 5w20 in an engine that recommends 5w30, and you can use 5w30 in an engine that recommends 5w20. It's not a big deal. As long as your not making a dramatic jump from 0w20 to 20w50, you'll be fine. What really matters is your oil change interval.
Can you mix 10w and 5W oil?
Yes, you can mix 5W and 10W oil, but it is not recommended. Mixing them can result in an oil with a viscosity that is somewhere between the two, which may not provide the optimal protection your engine was designed for, especially during cold starts. It's best to use the oil weight recommended in your vehicle's owner's manual.
When mixing is a temporary solution
- In an emergency: If you need to top off your oil and the wrong type is all you have, it's usually fine for a short period.
- To reduce risks: To avoid a larger jump in viscosity, it's better to mix two oils that are close in grade, like 5W-30 and 10W-30, rather than a larger jump like 5W-30 and 10W-40.
- Do not mix in extreme climates: In extremely cold weather, the 10W oil might not circulate as quickly as the 5W oil, potentially causing some extra wear on startup. In extremely hot weather, the 5W oil might not provide as much protection.
What to do after mixing
- Replace the oil soon: You should get an oil change and replace the mixture with the correct oil type as soon as possible.
- Check your owner's manual: Always refer to your vehicle's owner's manual for the correct oil viscosity and type.
- Consider synthetic vs. conventional: Do not mix conventional and synthetic oils.
Can you put 10W40 in a 5w20 engine?
No, you should not put 10W-40 oil in a 5W-20 engine because the thicker oil can lead to inadequate lubrication during cold starts and potential long-term engine damage. You should always use the oil viscosity recommended in your vehicle's owner's manual, as engineers designed the engine with that specific oil type in mind.
Risks of using 10W-40 in a 5W-20 engine
- Poor cold-start lubrication: 5W-20 is a thinner oil, designed to flow quickly and lubricate the engine parts as soon as it starts, especially in colder temperatures. 10W-40 is thicker and will not flow as easily, which can lead to increased wear on components like pistons during startup.
- Reduced fuel efficiency: A thicker oil requires more energy from the engine to pump throughout the system, which can result in a slight decrease in fuel economy.
- Potential for damage: Using the wrong oil can contribute to increased engine wear, especially in modern engines that are designed with tight tolerances and rely on specific oil pressure for systems like VVT (Variable Valve Timing) and chain tensioners to operate correctly.
What to do
- Consult your owner's manual: Always check your owner's manual for the manufacturer's recommended oil viscosity.
- Drain and replace: If you have already put 10W-40 in, it's best to drain the engine and replace it with the correct 5W-20 oil as soon as possible to prevent potential issues.
Is it okay to mix two types of engine oil?
Some of those additives work independently. Some are synergistic. And function together to accomplish a goal and some actually compete for surface area on internal. Parts.
