Does it matter what weight oil you put in your car?

Yes. The viscosity grade you use matters for cold starts, fuel efficiency, high-temperature protection, and overall engine longevity. Following the manufacturer’s recommended weight is the safest choice, while using an oil that's too thin or too thick can increase wear or reduce performance.

Understanding oil weight and why it matters

Oil weight, or viscosity, is rated by SAE numbers such as 0W-20 or 5W-30. The first number with W indicates cold-temperature performance; the second number indicates viscosity at operating temperature. The right grade helps maintain a protective lubricating film across conditions.

Common viscosity choices and what they mean

• 0W-20: Common in newer, fuel-efficient engines; provides good cold-start protection and adequate high-temperature performance.


• 5W-30: A versatile, widely recommended grade for many climates and engines; balances cold starts with protection during operation.


• 10W-30 / 10W-40: Found in older engines or hotter climates; may offer stronger film at high temperatures but can increase startup friction.


• 5W-40 / 0W-40: Used in some turbocharged or performance engines; broad operating range but not always necessary for every car.


• 15W-40 and higher: More typical in heavy-duty diesel or very hot climates; less common in modern gasoline engines.

Concluding: The appropriate weight depends on your car’s design, climate, and driving style. Using the wrong grade can increase wear, reduce efficiency, or cause startup issues. Always consult the owner’s manual or your dealer for the recommended range.

Why the manufacturer’s specifications matter

Automakers test engines with specific oil weights to ensure proper lubrication across temperatures and loads. The recommended weight accounts for clearances, oil pump pressure, piston rings, turbochargers, and emission controls. Using a significantly different viscosity can affect lubrication pressure, start-up wear, fuel economy, and warranty coverage.

Scenarios where a different weight might be considered

In some climates or driving conditions, a slightly thicker or thinner grade within the same family (for example, 0W-20 vs 5W-30 within the manufacturer’s guideline) may be acceptable. However, routine deviation from the OEM grade should be avoided unless your manual explicitly allows it, and you may need to adjust oil-change intervals accordingly.

Always verify climate, engine design, and service history; consult a professional if unsure.

Tips for choosing and maintaining oil weight

Follow these practical steps to ensure you’re using the correct weight and maintaining oil health across seasons and driving conditions.

• Check the owner’s manual and the oil cap or dipstick for the recommended viscosity and specification (e.g., API SP, ILSAC GF-6, or ACEA rating).


• Look for OEM-approved viscosity ranges on the label or manufacturer service site.


• Consider climate and driving: cold climates favor lower cold-start numbers (e.g., 0W), hot climates may tolerate higher high-temperature grades (e.g., 30, 40).


• Be mindful of engine age and condition: high-mileage engines may benefit from certain viscosities if recommended by the manufacturer.


• Avoid mixing different viscosities; stick to the recommended range.

Conclusion: The right oil weight matters for engine protection, efficiency, and longevity. When in doubt, refer to the manual and consult a mechanic or dealer.

Summary

Oil weight is a key maintenance detail. Use the viscosity the manufacturer specifies, consider climate and driving style, and monitor oil quality and changes. With the correct weight and regular maintenance, you protect engine health and performance.

What happens if I use 5W-40 instead of 5W30?

Putting 5W-40 instead of 5W-30 will result in slightly decreased fuel efficiency and potential performance issues, but is unlikely to cause major damage, especially in warmer climates or for older engines. The main difference is that 5W-40 oil is thicker at operating temperature, which can slightly increase friction and oil pressure. It's always best to follow the manufacturer's recommendation, which is found in your owner's manual or on the oil fill cap. 
What to expect

• Slightly reduced fuel economy: Thicker oil creates more drag on engine components, leading to a small decrease in fuel efficiency. 
• Potential performance impact: A thicker oil may slightly reduce high-end horsepower. 
• Increased oil pressure: 5W-40 has higher viscosity, which can lead to higher oil pressure readings. 
• Potential benefit for older engines: A thicker oil may be beneficial for older, high-mileage engines that have increased wear and tear, as it can help maintain oil pressure. 
• No harm in cold starts: The "5W" rating is the same for both oils, meaning both will have similar flow characteristics during cold starts. 

When to be cautious

• Modern engines: Modern engines are designed with precise tolerances, and using a thicker oil can cause problems that the manufacturer did not design for, so it is best to stick with the recommended 5W-30. 
• Warranty issues: Using an oil that does not meet the manufacturer's specifications can potentially void your warranty. 
• Stop-and-go traffic: If your engine runs hot in stop-and-go traffic, the thicker oil could be a benefit. 

Best practices

• Check your owner's manual: The manufacturer's recommendation is the safest guide. Check the manual for the correct oil viscosity and specifications. 
• Don't switch for "just because": There's no need to switch from 5W-30 to 5W-40 unless you have a specific reason, such as a high-mileage engine or a preference for heavier oil in a hot climate. 
• Stick to a single type: If you do switch, make sure to use the new oil at your next oil change, as mixing different viscosities is not ideal. 

Does it really matter what oil I put in my car?

Yes, it matters what oil you put in your car; using the wrong type can lead to engine damage, reduced performance, and a shortened lifespan. Always use the oil viscosity and type (conventional, synthetic, or blend) recommended in your car's owner's manual, as different engines require specific lubrication properties. 
Why the correct oil is important

• Lubrication: The right oil lubricates moving engine parts to minimize friction, which is essential for preventing wear and tear. 
• Protection: It protects against excessive heat and sludge buildup, which can impair engine function over time. 
• Performance: Using the correct oil maintains engine performance and can even improve fuel efficiency. 
• Warranty: Using the wrong oil may void your vehicle's warranty, especially with many newer vehicles that require full synthetic oil. 

How to choose the right oil

• Check your owner's manual: This is the most reliable source for the specific oil viscosity grade (e.g., 5W-30, 10W-30) and type (conventional, synthetic, blend) your engine requires. 
• Consider the viscosity numbers: The numbers on the oil container indicate its viscosity. The first number (e.g., the "5" in 5W-30) refers to how the oil flows in cold temperatures (W = Winter), and the second number (e.g., the "30") refers to how it flows when the engine is hot. 
• Look for specific certifications: Your manual might also list specific certifications or specifications your oil needs to meet. Look for these on the bottle. 
• Don't mix: While occasionally mixing oil types won't cause immediate damage, it's best to stick to one type. If you need to top off, only do so with the exact type specified in your manual. 
• When in doubt, ask: If you're unsure, ask a dealership or mechanic what oil your car needs. 

What happens if I put 5W-20 instead of 5W30?

Putting 5W-20 instead of 5W-30 oil means you are using a slightly thinner oil when the engine is hot. This can lead to slightly less engine protection, especially in hot weather or during heavy use, and a minor reduction in fuel economy. However, for most modern cars, the difference is minimal and there is no need for immediate concern unless your vehicle is a high-performance model, in which case you should consult your owner's manual and consider changing to the correct oil as soon as possible. 
Potential effects of using 5W-20 instead of 5W-30

• Reduced protection: 5W-30 is thicker at operating temperature, which provides a slightly better protective film on engine parts. The thinner 5W-20 may lead to a minor increase in wear. 
• Increased fuel economy: Because 5W-20 is thinner, it requires less energy to pump, which can result in a small improvement in fuel efficiency. 
• Impact on high-performance engines: Some high-performance engines have tight tolerances and are designed for thinner oil. However, putting 5W-20 in an engine that specifies 5W-30 can be problematic for some modern high-compression engines, potentially leading to bearing failure if driven hard, says Quora. 
• Warranty concerns: If your car is under warranty, using the wrong oil could potentially void your warranty in case of a powertrain failure. 

What you should do

• Check your owner's manual: The safest bet is to always use the oil recommended by your car's manufacturer. 
• If the wrong oil was added during an oil change:
  • Modern cars: For most modern cars, this will likely not cause any immediate damage, but you should have the oil changed to the correct weight as soon as possible. 
  • High-performance cars: If you have a high-performance vehicle, change the oil to the recommended weight immediately to avoid potential damage. 
• If you just added a small amount of 5W-20 to an engine that calls for 5W-30: This is generally not an issue. 
  • For most modern cars, you can continue using 5W-30 as recommended in your owner's manual. 
  • However, if you have a high-performance vehicle, check your owner's manual to see if you need to change the oil to 5W-30 immediately. 

What happens if you accidentally put 10W30 instead of 5W30?

Using 10W-30 instead of 5W-30 is generally acceptable for many vehicles, especially in warmer climates or with older engines, as both oils have the same viscosity at operating temperature. However, 5W-30 is recommended for cold climates because its lower "winter" viscosity allows it to flow more easily during cold starts, reducing initial engine wear. Swapping to 10W-30 in a vehicle designed for 5W-30 can lead to slower oil flow and potential damage if it's frequently exposed to temperatures below freezing. 
When it's likely okay

• Warmer climates: If temperatures consistently stay above 0∘F0 raised to the composed with power cap F0∘𝐹 (-18∘Cnegative 18 raised to the composed with power cap C−18∘𝐶), the thicker 10W-30 is less likely to cause problems. 
• Older engines: A slightly thicker oil can sometimes help older, high-mileage engines that might burn oil or have larger internal gaps due to wear. 
• Moderate driving: If you don't frequently drive in extreme cold, the difference will be minimal. 

When to avoid it

• Cold climates: In regions where temperatures frequently drop below freezing, the 10W-30 will be thicker and may not lubricate the engine as quickly on a cold start, which is when most engine wear occurs. 
• Modern engines: Newer vehicles with specific requirements for oil pressure, like those with variable valve timing (VVT), should stick to the manufacturer's recommendation to avoid potential issues. 
• When the owner's manual specifies 5W-30: The safest and best practice is to always use the oil viscosity recommended in your vehicle's owner's manual, as it was specifically chosen for that engine's design. 

This video explains the difference between 5W-30 and 10W-30 motor oils: 53sThe Savvy ProfessorYouTube · Apr 19, 2023
Potential signs of a problem
Tougher cold starts, Ticking or tapping noises after startup, Higher than normal oil pressure reading, and A noticeable drop in fuel economy.