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How to mix different oils correctly?

The question about mixing oils of different manufacturers and viscosities causes a lot of disputes among drivers. The quick answer is that you can mix, but stick to some rules, which we will consider below. If you mix other brands or viscosities correctly, the engine will not suffer at low mileage. I argue my position based on information on the web, personal observations and opinions of other experts.

Can I mix oils from different manufacturers?

It should be noted at once that the problem of oil mixing is not only in the process of oil replenishment but also in its replacement. Any car starts its use, coming out of the conveyor line with the oil poured into the engine at the factory – it is the engine manufacturer’s recommended product, officially approved for the first fill. Then the service maintenance requirements stipulate the use of other specific brands of oil.

So the problem is that when you drain the oil some amount is left in the engine anyway – about 10% of the total volume. That is mixing of oil happens anyway, even if you don’t refill another brand of oil during the recommended mileage, mixing still happens after the change. For this reason, manufacturers recommend using the same brands of oil or at least choose the same viscosity and properties.

Each oil has its own base, additive package, and properties. Mineral, synthetic, or semi-synthetic bases are mixed with additives in specific proportions to achieve the required performance properties. Each group differs in composition, even the same manufacturer’s lineup of oils can vary dramatically.

The short answer to whether you can mix oils from different manufacturers: extremely undesirable, but permissible. There are a number of API standards that require all oils that have passed them to be compatible with other products that have a similar specification. The danger of haphazardly mixing different oils is the potential for chemical reactions, sludge, foaming, and other phenomena. What you should not do is mix mixtures with and without molybdenum, because of the very large difference in the additive package.

It is allowed to pour any oil, regardless of its viscosity and class, but only for an emergency for a short mileage, after which you need to completely replace the lubricant and oil filter, reduce the mileage interval of new oil by 2 times (not necessarily, but desirable), because, as we said above, 10% of the mixture with unpredictable reactions will remain in the engine.

Can synthetic, semi-synthetic, and mineral oils be mixed?

You can, but not in all combinations, here are the basic permissible compatibility of oils by type:

  1. Mineral and semi-synthetic. If mineral oil is currently filled, you can dilute it a bit with semi-synthetics.
  2. Synthetic and semi-synthetic – you can mix viscosities 5W-40 and 10W-40, the resulting mixture will be within the viscosity of SAE 6W-40 – 8W-40. It is best if a higher quality lubricant is poured into an existing oil. That is if synthetic is poured into the main volume of semi-synthetic, if vice versa, the situation is critical.
  3. Only synthetics are filled with synthetics and the oil quality is not reduced. From the variety of synthetic compositions choose the same base.
  4. You can be guided by the following scheme for safe mixing: 15W-40 (mineral) + 15W40 (semi-synthetic) or 10W-40 (semi-synthetic). 10W-40 (semi-synthetic) + 5W-40 (synthetic). 5W-40 (synthetic) + 5W-30 (synthetic).

It is best to combine oils of different types from the same manufacturer, as the base technology is the same, i.e. the final product has a similar chemical composition. If you are planning to switch from synthetic to semi-synthetic or mineral or make any other combination, it is better to use the same brand for a couple more cycles.

If a refill is needed, it is better to take the same manufacturer’s products of a different type, viscosity, etc., than to use a different brand of oil that fits the parameters. But always such mixtures should be of short duration.

Can different viscosities of oils be topped up?

It is better to use the same viscosity or a different viscosity, but of the same manufacturer, their formulations are designed to be compatible with each other with the least risk. If a different grade of oil is added, the overall viscosity drops, which must be taken into account.

For example, if 5W-40 is added to a 10W-40 mixture, the resulting mixture will be 6W-40 or 7W-40 depending on the amount of oil added, meaning the resulting viscosity may not match the vehicle’s manual. If a complete change of lubricant in the system to a different viscosity, it is better to use the same brand, keeping in mind that 10% remains in the system.

Best engine oil for VW

Is it possible to refill other oil: to summarize

  1. If you summarize all the recommendations and draw conclusions from all the disputes on this subject, you can derive such a number of tips, regarding the refilling of the engine with other oil:
  2. It is best to refill oil of the same brand, but with different viscosity, it guarantees the absence of negative phenomena or their minimum amount. But it is better to take the viscosity according to the season, so as not to lower it, for example, to the summer level in winter. Usually, manufacturers use the same additive package in all their oils, that is why there should not be any reaction to mixing.
  3. You can refill mineral oil with semi-synthetics, but not pure synthetics. Some additives do not dissolve in mineral oil and may precipitate out. Diluting synthetics with mineral oil is especially dangerous.
  4. If you have to top up with another manufacturer, it’s better to use the product which is similar in its class and level of performance properties. According to the API and ACEA requirements, oils of the same class are very close to each other in their properties and composition, and they can safely replace each other or mix. Car manufacturers’ approvals also play an important role in this issue. But this mixture should be replaced as soon as possible.
  5. If you could only find an oil with a lower class and properties at hand, it is better to reduce engine load and not to drive at high speed.
  6. If less viscous oil was added, it may increase the burn rate, so the level will have to be monitored more often.


So, can oils be mixed? The answer is yes, with a few caveats. Always do your research and test mixes in small batches before using them on expensive equipment or vehicles. Be sure to use the same weight or volume of each oil when creating your mix, and avoid mixing different types of oils (synthetic, mineral, semi-synthetic). As long as you follow these simple tips, you should be able to safely mix various oils together without any problems.

We hope this article will help you choose the right oil for your car. Take care!)

Related posts:

How Much Oil Does My Car Need
What Is Multigrade Oil
What Is Engine Oil Volatility
What Is Synthetic Oil

Frequently Asked Questions (FAQ)

When it comes to mixing oils from different manufacturers, there can be potential issues in compatibility. Before attempting to mix oils, it is important to consider the properties of each oil, such as viscosity and additives. In general, motor oils that are manufactured with similar base stocks (the primary component of lubricating oil) can usually be safely mixed together without creating any major problems.
Mixing essential oils is a great way to create more customized fragrances and blends. When it comes to blending, some essential oils play nicely with each other while others may not be compatible. It’s important to know which oils are best suited for mixing so that you can make the most of your aromatic experience.

Best practice is to use all the same brand of oil when changing your engine’s oil. Different synthetic oil brands may contain ingredients that are incompatible with each other, so it’s important to check the compatibility before opting for this approach. If you do decide to mix two different brands of synthetic oil, make sure they both meet applicable specifications and have similar viscosity ratings.



David Muench (Carnes Mechanical)

David Muench (Carnes Mechanical)

Hey, I’m David. I’ve worked in a cars store for 6 years. I write reviews and guides, helping people to choose the most suitable technicals and best product for them. I’m happy to finally share my knowledge of the industry here, on CarnesMechanical.

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