Mix Oil Weights: Blending Oils of Different Weights

The question of whether it is safe to mix oil weights or not has been a hot topic among car enthusiasts. This question usually comes up when one’s vehicle is low on motor oil and the person doesn’t have enough of the same oil weight to replenish but has another weight of oil in store.

Mix Oil Weights

Thus, they are faced with two different motor oil weights and they wonder whether it would impede the efficiency of the engine.

This article will discuss the details of the answer to this question and cover the different types of engine oil as well.

Is It Good To Mix Oil Weights?

No, it is not good to mix oil weights because of several factors including the difference in weight of both engine oils. However, mixing oil weights is not a bad idea if you have run out of oil and do not have any other choice.

Doing this is not healthy for the engine in the long run, but having enough motor oil in your vehicle is better than having none or an insufficient amount. This is because different engine oils are designed to satisfy the requirements of specific engines.

Thick motor oils are designed for the engines of heavy-duty cars and machines while lighter engine oils are made for light vehicles like sedans and SUVs. Thus, if you mix the two during an oil change, you’re likely to hamper the engine’s performance and destroy some parts of the engine.

– Oil Weights

The weight of an engine oil is its ability to flow at a specific temperature of 212 degrees Fahrenheit. Technically, the weight of an oil is termed oil viscosity and it comes with specific figures indicating it. Examples of such figures include 5W-30, 10W-30, 15-40, etc.

Oil Weights

– Understanding Oil Weight Figures

Car engine oil containers have the weight or viscosity of the oil written on them so drivers can know the exact oil to purchase for their vehicles. A motor oil container labeled 5W-10 means that at colder temperatures, that is “W” for winter, the oil has a viscosity of 5. The 10 after the dash indicates that at hotter temperatures (especially in the summer), the oil has a viscosity of 10. Note that more viscosity means that the motor oil is thicker while less viscosity indicates that the oil is lighter.

Also, keep in mind that hotter oils flow more easily than colder ones, thus the numbers by the “W” will always be less than the numbers after the dash (-). Therefore, what all these mean is that vehicles that require a gasoline engine oil of 10W-30, do not use a 5W-10. Using a 5W-10 will be too light and cause the different engine parts to rub against each other. This can impede the performance of the engine, or worse still, cause wear and tear in some parts of the engine.

Conversely, using a 10W-30 graded oil in an engine that requires a 5W-10 oil will be too thick for the engine. This might increase the friction between the moving parts, forcing the engine to exert extra energy to work. Therefore, it is recommended to you check the label of the engine oil and the car’s manual to know which engine type is better for your vehicle. This will save you a lot of repair work down the road as the engine performs optimally and efficiently.

– Mixing the Same Viscosity Motor Oils But Different Brands

Generally, this shouldn’t be harmful to your engine because of international regulations on the manufacturing of synthetic engine oils. The regulations ensure that engine oils of the same viscosity are similar and operate the same way regardless of the brand.

Though many engine oil-producing companies advertise that their products are a step above the rest, the truth is that the differences are minute. Therefore, you can blend motor oils of the same viscosity but different brands, and you shouldn’t have any major problems.

However, take note that due to the minute differences, there is still a chance that certain substances in oil “A” may not react well with others in oil “B.” Therefore, we recommend that you blend oils of the same viscosity but different brands with caution. To be on the safer side, always use the same brand of oil and avoid mixing them.

In short, oils of the same weight but different brands can be mixed but caution should be exercised. There may be some additives in one oil that may not gell well with another additive in the other oil, which may disrupt engine performance and lead to costly repairs.

– Mixing Different Oil Types

It is not advisable to mix motor oil types as you may experience less engine protection from the two types.

You can do it on a short-term, if you want to top up your oil for use until the following day when you can get the right oil for your engine.

However, this shouldn’t be common practice. Also, take note that mixing oil weights on a motorcycle is also not advisable.

– Mixing 30 and 40 Weight Oil

Oils of different weights shouldn’t be mixed as it may possibly affect the performance of the engines.

Mixing 30 and 40 Weight Oil

Oils of different weights flow differently and degrade differently, thus it is a bad idea to mix them.

– Mixing 5W30 and 10W40 Oil

You can mix these oils but it could affect engine performance and fuel consumption as both oils have different viscosities and could stress the engine.

Make sure you are well aware of the risks before doing something like this.

What Are the Reasons Why Engine Oils of Different Weights Shouldn’t Be Mixed?

The reasons why engine oils of different weights shouldn’t be mixed include the difference in motor oil viscosity, synthetic engine oil additives, and the possibility of creating an unstable mixture. As much as possible, make sure to stick to only one engine oil for each vehicle.

If you’re still not convinced that mixing oils of different weights is not helpful for your engine, then we still have a few reasons that may do the trick.

– Synthetic Engine Oil Additives

Synthetic oil brands have unique formulae and additives that enhance the performance of their products. Therefore, there’s no way of knowing whether the different additives from different brands will interact positively or negatively. You can blend two oils and their additives may enhance the car’s performance or affect the engine’s efficiency. Our research showed that even switching between brands of engine oils with the same viscosity can be detrimental.

Synthetic Engine Oil Additives

This is because the components in the old oil could react with that of the new oil and this mixture could corrode parts of the engine. Therefore, if mixing oil of the same weight but different brands has the potential to cause damage, how much more mixing oils of different weights? Since mixing oil weights synthetic is not advisable, It is best to use engine oils of the same viscosity and brand to avoid impeding your engine’s performance.

– A Different Motor Oil Viscosity

Mixing two oils of different weights could produce a new viscosity which is in the region of SAE 40. This oil is thicker and is suitable for the winter season or colder months. However, using it can cause too much heat buildup and create energy loss. Also, the engine may exert extra energy to get the vehicle moving due to the thickness of the oil and this would lead to more fuel consumption. 

Also, thicker oils are poor at heat conduction, which can speed up the oxidation of the engine lubricant. This can cause more sludge in the engine, hampering its performance.

– The Mixture May Prove Unstable

Blending oils with different viscosity may result in an unstable mixture that could hurt the engine. This could be down to the different additives not gelling well together which could mess up the viscosity.

As you already know, viscosity is everything when it comes to engine oils, thus a messed-up viscosity could be detrimental to the car’s performance. Therefore, get engine oils that are of the same brand and viscosity to preserve the life of your engine.

Would Mixing Oils Improve Engine Efficiency?

No, mixing oils will not improve engine efficiency, regardless of what you might have heard or read. This is because oils of the same viscosity have similar qualities, which is the same as using one brand of engine oil. Therefore, always stick to the same brand of oil. Also, driving without oil is extremely dangerous to your car!

What Are the Types of Engine Oils?

The types of engine oils include conventional, high-mileage, synthetic and synthetic-blend. These oils have specific functions and are suitable for particular engines, thus they perform best when used appropriately. For example, the engine oil for a light-duty car shouldn’t be used in a heavy-duty car.

– Mineral/Conventional Oil

This is the basic form of engine oil derived from refined petroleum oils which are treated to make oil kinds of engine oils. This type of oil is cheaper than the rest and is perfect for light-duty older car models with simple designs and motorcycles. However, the downside to this type of oil is that it doesn’t protect against heat caused by friction and can easily break down in high temperatures. 

This type of oil is inefficient in colder temperatures and needs frequent replacement, or else the engine will suffer. They need to be replaced every 5,000 kilometers.

– High-Mileage Oils

This type of oil has additives that enable it to cater to older engines. Some of these additives include conditioners, detergents, antioxidants, friction, wear and seal swell.

These additives prevent engine breakdown, minimize harmful emissions, and decrease oil consumption. High-mileage oils are thicker and contain a viscosity modifier, which keeps their thickness for a long time.

– Synthetic Motor Oils

Synthetic oils, also known as mand-made oils, are highly refined mineral oils that contain artificial compounds that are meant to improve the performance of engines. Various man-made oil-producing companies have their “secret” additives that they claim are perfect for vehicles.

Synthetic Motor Oils

However, take note that these “secrets” are guided by international conventions, thus there would only be little differences. Man-made oils are more expensive and better engine oils in the long term.

– Synthetic Blend/Semi-synthetic Motor Oils

This is a combination of synthetic and mineral oils to give consumers the affordability of mineral oils and the efficiency of man-made. Semi-synthetic engine oils are better than mineral oils and protect the moving parts of the engine better.

Usually, a small amount of man-made oil is added to the mineral oil to enhance its viscosity and resistance to wear in hotter temperatures. The only downside is that they don’t perform at the level of a completely man-made oil.

How Can Blending Oils of Different Weights Affect the Oil Capacity of a 6.7 Cummins Engine?

Blending oils of different weights can have an impact on the cummins oil volume capacity of a 6.7 Cummins engine. The viscosity of the blended oil will determine its flow and ability to lubricate engine components effectively. It is crucial to consider the manufacturer’s guidelines and recommendations while selecting oil weights to maintain optimal performance and avoid any potential damage to the engine.


This article has discussed, in detail, whether two oils of different weights can be mixed or not.

Here is a recap of all that we’ve read so far:

  • Mixing two oils of different weights would not damage the engine nor would it improve its performance but it is advisable not to mix them.
  • Two oils with different viscosities can stress the engine and lead to high fuel consumption as the engine stresses to keep the moving parts going.
  • Oils of the same brand but different viscosities can also be mixed but this should only be done in the short term.
  • The best practice is to check your car’s manual to identify the recommended oil for your car so you can avoid using the wrong type of engine oil.
  • Using the incorrect type of engine oil can affect the engine’s performance and even cause wear and tear on the moving parts.

Our final thoughts are that you can use engine oils of different brands but the same weight due to the international regulations governing the production of engines. This is because there are only minute differences between oils of the same viscosity but different brands. You can go for an oil change at Walmart which is a very quick process!

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