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10w30 vs. 10w40

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10w30 vs. 10w40 the difference

Selecting the appropriate engine oil for your automobile is significantly influenced by your comprehension of oil viscosity ratings. You will frequently encounter 10W30 and 10W40, two prevalent multi-grade oils, in your search for the ideal oil.

At a casual glance, these oils may appear to be identical, but they have distinct properties and are suited to different scenarios. This article aims to provide a detailed examination of the 10W30 and 10W40 motor oils, assisting you in making an informed decision for your automobile’s needs.

Our Recommendations:

Based on our hands-on experience and customer reviews, these two products are the best for each type of motor oil.

The left side is 10w30, and the right side is 10w40.

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10w30 vs. 10w40: What’s the difference?

10w30 vs. 10w40: What's the difference?

The difference between 10W30 and 10W40 motor oils lies in their viscosity at high temperatures (100°C or 212°F), as indicated by the second number in their ratings. Both oils have the same viscosity at cold temperatures (’10W’), ensuring good flow for start-up lubrication. However, at high temperatures, 10W30 is thinner than 10W40.

The higher viscosity of 10W40 at these temperatures means it can provide better protection in hotter climates or under more severe driving conditions.

Conversely, 10W30, being thinner, offers slightly better fuel efficiency [source] as it requires less energy to circulate through the engine. The choice between the two should depend on your vehicle’s specific needs and the manufacturer’s recommendations.

I wouldn’t recommend going thinner just to eek out a bit more mileage on your vehicle.

In terms of cost, 10w30 will be cheaper, but it won’t be by a lot.


Viscosity is a measure of a fluid’s resistance to flow. In simpler terms, it can be thought of as the ‘thickness’ or ‘stickiness’ of a fluid. The higher the viscosity, the thicker the fluid is and the slower it flows. Conversely, a fluid with low viscosity is thinner and flows more easily.

Sadly, it isn’t as simple as “high viscosity good and low viscosity bad”.

What happens if you use 10w30 instead of 10w40?

If you use 10W30 instead of 10W40, you may not notice an immediate difference under normal driving conditions, especially in moderate climates. Both oils have the same viscosity at cold temperatures, so you won’t notice any difference during the winter or cooler months.

However, at high operating temperatures, 10W30 is thinner than 10W40. If your vehicle typically operates under high-stress conditions such as towing heavy loads, frequent stop-and-go traffic, or if you’re driving in a hot climate, 10W30 may not provide the same level of protection as 10W40.

The thinner oil could potentially lead to increased engine wear over time under these conditions.

oil viscosity

And, of course, as discussed before, 10w30 can have increased fuel savings.

It’s essential to note that you should always use the oil viscosity recommended by your vehicle’s manufacturer. Using an oil with a different viscosity than recommended could potentially impact your vehicle’s performance and engine life and even void your warranty. If you’re considering using a different oil than recommended, it’s best to consult a trusted mechanic or vehicle manufacturer.

Which to use in the Summer?

10w30 vs. 10w40 in the summer

The choice between 10W30 and 10W40 motor oil during the summer depends on a few factors:

  1. Vehicle Manufacturer’s Recommendations: Always follow the oil viscosity recommended in your vehicle’s owner’s manual. The manufacturer’s recommendations are based on extensive testing and should be your primary guide.
  2. Driving Conditions: If you frequently drive under high-stress conditions (towing heavy loads, high-speed driving, frequent stop-and-go traffic) or in particularly hot climates, a 10W40 oil might be more suitable due to its higher viscosity at high temperatures, offering better engine protection.
  3. Engine Age and Condition: Older engines or those with high mileage may benefit from a thicker oil like 10W40, which can help prevent oil leaks and provide better protection for worn engine parts.

Remember, the ’10W’ in both 10W30 and 10W40 means that they have the same viscosity at cold temperatures and should provide similar protection during cold starts.

The main difference lies in their performance at high temperatures (100°C or 212°F), which is likely to be reached during summer months or under high-stress driving conditions. Here, 10W40, being thicker, might offer better protection.

Again, it’s crucial to consult with a trusted mechanic or your vehicle manufacturer before switching oil types to ensure your vehicle’s best performance and longevity.

10w30 Motor Oil

10w30 Motor Oil


  1. Better Fuel Efficiency: Due to its lower viscosity at high temperatures, 10W30 oil can improve fuel efficiency slightly as the engine requires less energy to pump the oil.
  2. Suitable for Moderate Conditions: It is generally sufficient for regular driving conditions and moderate climates.
  3. Wide Application: 10W30 is often recommended for a broad range of vehicles, making it a versatile choice.


  1. Lower High-Temperature Viscosity: At high temperatures, 10W30 oil is thinner than 10W40, which might not provide adequate protection for engines operating under high stress or in hot climates.
  2. Not Ideal for Heavy Loads: If your vehicle often tows heavy loads or operates under high stress, 10W30 might not offer sufficient protection.

10W30 motor oil provides good viscosity performance at both low and high temperatures. At low temperatures, it flows well, ensuring the engine parts are lubricated during cold starts. At high temperatures, the ’30’ means it is slightly thinner than a ’40’ oil.

This makes 10W30 a suitable choice for moderate climates and regular driving conditions. It provides adequate protection in most scenarios and is often recommended for a wide range of vehicles. It also offers slightly better fuel efficiency compared to thicker oils, as it requires less energy to pump the oil through the engine.

Pennzoil makes a great 10w30 motor oil product, and I personally recommend purchasing from them if this is the type of oil you need. I purchased the 5-quart bottle on Amazon.

10W40 Motor Oil

10W40 Motor Oil


  1. Superior High-Temperature Performance: With higher viscosity at high temperatures, 10W40 oil can offer better protection under high-heat conditions, ensuring the engine stays well-lubricated.
  2. Great for Heavy-Duty Use: It is more suitable for vehicles that operate under heavy loads or stressful conditions, providing the necessary protection against wear and tear.


  1. Lower Fuel Efficiency: Due to its thicker nature at high temperatures, 10W40 oil might slightly reduce fuel efficiency compared to 10W30.
  2. Not Necessary for Regular Conditions: If your vehicle operates under normal conditions most of the time, the added protection of 10W40 might not be necessary, and a 10W30 oil could suffice.

10W40 motor oil, like 10W30, flows well at low temperatures, ensuring good start-up lubrication. However, the ’40’ signifies that this oil is thicker at high temperatures compared to 10W30. This characteristic makes it more suitable for high-temperature operations or for vehicles that typically operate under heavy loads.

This higher viscosity at high temperatures means that 10W40 can provide better protection in hotter climates or under more severe driving conditions. However, this also means it may slightly reduce fuel efficiency compared to 10W30 due to its increased resistance to flow.

Castrol has the best 10w40 I’ve found so far. I also found their content incredibly useful for understanding the difference between 10w30 and 10w40. Some of what is written here are from research I gathered from several of Castrol’s online resources.

Therefore, I do recommend purchasing from them if you decide to go with 10w40. They sell a 5-quart bottle on Amazon.


While you might be worried that the difference between 10w30 vs. 10w40 is a big one, the only real factors you should consider are the climate and the average use of your vehicle.

People living in hot climates or using vehicles for heavy-duty purposes should go for the 10w40. Conversely, cold or people who only use their vehicles for light transportation should stick with 10w30.

That all being said, your vehicle manufacturer or local mechanic should be the final say in the matter.

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