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Can You Mix Engine Oils?

One question that regularly circulates especially among the novice auto enthusiast is “Can you mix oils from different brands, categories, and viscosity?”

Knowing this might be extremely helpful to know especially if you have to urgently top up some motor oil into the engine in the middle of the road and you don’t have much of choice around you. So let’s find out.

Mixing motor oils might not be fatal for your engine but you need to follow some simple rules and be careful and observant until the next oil change.

Can You Mix Different Brands of Engine Oil?

Each product consists of a base and a set of additives that form the oil base and the individual qualities of the particular engine oil.

The possible incompatibility of the bases from different manufacturers is the first problem – different manufacturers have different technologies and ways of producing the bases, which may eventually have different physical properties (this applies especially to synthetic motor oils).

Due to the differences in the bases, problems may still occur when mixing relatively identical synthetic products but from different manufacturers.

And this is because a much more essential factor is playing here – the additives.

Let’s put it that way: to achieve the same viscosity and temperature characteristics in a 10W40 oil, for example, the manufacturers are solving completely different problems determined by the properties of their own base.

And the set of additives that finally solves their problem leads to a completely different standard.

When two of these different sets of chemical elements, which are additives, start interacting with each other there is no guarantee of how will they react, and if they will be compatible.

So in other words, mixing /topping up motor oils from different brands/ manufacturers should be avoided (if possible), as their compatibility cannot be guaranteed.

The main problem is the inability to predict the compatibility of the oil package included in the oil composition, while base oils are largely compatible.

It is the components (chemicals) that make up the additive package that may be incompatible with each other.

Incompatibility in motor oils may look different:

  • Sudden change in transparency, foaming or darkening of oil after mixing
  • Partitioning of the layers or sludge formation
  • Sudden oxidation of the mixture oils – a formation of greasy deposits in the engine.

But very often due to different circumstances you are forced to top up with oil from a different manufacturer. In that case, you would be satisfied with what the nearest garage or car parts store has to offer.

What should you do when mixing engine oil from different manufacturers?

Engine oils rack

The first and simplest advice is to choose an oil from a well-known and reputable manufacturer.

Second, choose a product that has similar physic-chemical and performance characteristics to the one you already have in the engine.

For example, in the engine, you have conventional oil with 15 W – 40 viscosity and API CH -4 specification. It is desirable to select an engine oil with similar characteristics: same viscosity (15 W – 30, 15 W – 40, 10 W – 40) and comparable specification.

Synthetic motor oil with a viscosity of 5W – 30 is not a good choice in our example. Simply, always be guided by the simple logical consideration.

Three unwritten rules about mixing engine oil from different brands

  • If you topped up below 10% of the total engine oil capacity, then it is considered that no problems will occur
  • If you topped off from 10% to 30% it is advisable to reduce the remaining miles to the next oil change twice. For example: you have 4000 miles to the next oil change but meanwhile, you topped up about 30% different oil so it is recommended to change it after 2000 miles
  • If you topped up more than 30%, it is advisable to have an engine oil change as soon as possible.

Our comparison and reviews on the best synthetic motor oil you can read here.

Can You Mix Different Types of Engine Oil?

You already know what happens if you mix engine oils from different brands so let’s see how your engine feels when you mix those from different types.

The main problem is that conventional oils, unlike synthetics, do not have a stable viscosity and specific additives have to be used. Furthermore, it is not clear how they will interact with each other over time.

So in simple words, it is not recommended to mix different types of motor oils because it is difficult to predict what exactly will happen. The main problems that may occur are the following:

  • Engine contamination – slag deposits and more
  • Sedimentation of some of the additives or reduction of their effectiveness
  • Increasing the viscosity to its full thickening and clogging the main oil paths.
  • The result may be lamentable – in the worst scenario, you might need a complete engine rebuild.

Why You Mix Engine Oils?

It seems clear why it’s not recommended to mix different oils but why does this question constantly arise? The reasons are simple:

  • Sometimes you are forced to top up with just whatever engine oil is available around bullet point
  • Globalization and unification can have a positive effect on the oil market: engine oil bases, and in particular, additives, are produced by a small number of manufacturers, which significantly reduces their compatibility problems. The good thing is those producers are also aware of the problem and are constantly trying to resolve it.

Here every driver makes his own decision: whether he is willing to take the risk for the sake of short-term money saving/time/ problem reduction or not. If yes – read the general recommendations that are intended to reduce the likelihood of a problem:

  • Never mix synthetic with conventional oils. If you are forced to do so just to keep your vehicle running for a while, make sure to change your oil as soon as possible.
  • As a last resort, you can mix one manufacturer, but of a different kind (for example, Mobil 5W30 synthetics and Mobil 5W40 synthetics).

Conclusion

Mixing different kinds and engine oil brands is not recommended as a general rule. But sometimes we are forced to do so when we are stuck in the middle of nowhere and we only have just a bottle of oil that somebody offered us.

To prevent costly repairs and save you time, we advise you to have an oil change and engine flush as soon as possible.

Some engines may carry on with the mixed oils but some can react negatively very soon. It all depends on the two oils that are mixed and their correction.

Jason Clark

Jason Clark

15 years of experience as a professional mechanic. Rally car test pilot and advisor.

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