Can You Mix Synthetic Oil with Regular Oil? Keep Your Engine Safe!

When it comes to mixing oil in our vehicles, there are clear cut rules to abide by. If you don’t abide by these rules, serious problems can happen.

Adding a splash of diesel to a gasoline-run vehicle can be disastrous, combining two different brands of coolant is inadvisable (to say the least), and playing the role of chemist by combining chemical automotive products is asking for trouble.

But what about lubricants? Specifically, can you mix synthetic oil with regular oil? Or should this be avoided at all costs?

The answer may not be as obvious.

Car owner filling up his vehicle's engine with motor oil

In this article, you’re going to learn whether or not you can mix conventional oil with synthetic oil, as well as all the reasons why people consider mixing these two types together.

By the time you finish reading this guide, you’ll know under what circumstances it’s a good idea to attempt mixing regular oil with synthetic oil and how to do it, and you’ll also discover when and why doing this is best avoided.

What’s the Difference Between Synthetic Oil and Regular Oil?

These motor oils are similar in some respects. They’re both petroleum-based lubricants, they both help moving parts in your engine run smoothly, and they both dissipate heat and clear debris while circulating.

Synthetic and regular oils are derived from the same source: crude oil. Both types contain additives that improve how they work.

These helpful compounds possess many beneficial properties, and you can consider them as the automotive version of supplements.

Certain chemicals within the oils are detergents that break down deposits and sediment. Others work to boost lubrication, fight corrosion and prevent foaming.

The main difference between the two is in the refinement process. The original fossil fuel, crude oil, is altered in specific ways depending on what it’s destined to become. This is true of any petroleum product.

For instance, waxes are subjected to a different process to distillates like diesel. Jet fuel doesn’t go through the same chemical and physical changes as petrochemical feedstocks.

Regular (Conventional) Oil

Let’s start with regular oil, which also known as conventional oil.

Regular oil tends to be heavier than its synthetic blend counterpart. Although it goes through refinement, the composition stays truer to the original fossil fuel. In this respect, it’s less environmentally friendly, producing more toxic emissions.

In the past, vehicles used to run exclusively on conventional lubricants. The scientific innovations that led to unconventional lubricants used today didn’t exist back then.

Synthetic (Unconventional) Oil

Synthetics, also known as unconventional oils, are made from ingredients that have been synthesized.

To clarify, synthesizing a certain chemical or compound doesn’t translate into fake ingredients. It just means that the substance has been chemically altered. In fact, many synthetics are made with petroleum products, most of which are modified.

In addition, synthetics go through more processing than conventional brands, which is done to remove impurities that can increase pollution or reduce lifespan.

They also tend to include a greater amount of additives. Unconventional lubricants are generally manufactured to deliver the best possible performance.

As you may expect, these benefits don’t come cheap, as there’s a lot of resources involved in manufacturing such products.

A bottle of synthetic oil will cost you more than the same quantity of conventional motor oil. They’re also lighter than their conventional counterparts, which is useful when the temperature drops.

Last but not least, you’re less likely to end up dealing with cold starts with synthetic motor oil, as opposed to using a denser lubricant.

Blended – the Best of Both Worlds

The third type of oil we can use on our cars falls within the category of unconventional oils, just like synthetic motor oils do. Blended brands are those that include both conventional and synthetic oils.

They have a wider range of viscosity, or weight, than full synthetics.

Also, a huge bonus is that blended brands tend to be less expensive.

Why Do People Mix Synthetic Oil With Regular Oil?

There are a few reasons why you might be thinking of mixing synthetic oil with regular oil and making your own blended combination.

These may include one or more of the following:

Running Low on Oil

Those of you who drive long distances on a regular basis and have high oil consumption needs may have experienced this situation multiple times before.

You’re on the road and notice you’re starting to run low on oil. Perhaps the gas station you stop at doesn’t stock the right brand that works best for your car, or your preferred lubricant is at home and not stored in the vehicle.

In these (and other) cases, there’s not really much choice but to combine synthetic base oils with regular ones.

Boost Performance

You may have heard that blending these two types can improve how your engine runs. The story goes something like this: Adding a dose of unconventional lubricant to your old, conventional lubricant will enhance performance.

Unfortunately, these fluids aren’t like vitamins where a little portion can go a long way. You can’t expect to notice much of a positive change unless you get the ratios absolutely right.

If you’re using fully synthetic, adding conventional engine oil is a bad decision to take. The former already has everything you could want in a lubricant, so why taint it with a lower-quality type?

Save Money

Unconventional oil brands are expensive, and you already know that by now. So, why not mix the more costly lubricant with a cheap conventional oil?

Hypothetically speaking, you can get the benefits without using an entire bottle of the expensive stuff. It’ll last you for longer, saving money in the long-term.

As with boosting performance, the idea may sound good in theory, but doesn’t work all that well in practice. In short, you’ll need to add a significant amount of synthetic engine oil to notice any appreciable difference.

Extended Lifespan

Conventional oil brands are more likely to degrade faster compared to unconventional varieties. Adding the latter to the former may extend overall lifespan, buying you time between each oil change interval.

We hate to burst your bubble, but the same issue comes into play if you’re trying to benefit from this. A large amount of synthetic oil would be needed in order to make a significant impact.

Change Brands

This is an age-old misconception about switching brands.

You might have heard or read somewhere that a slow transition between brands is required, where you have to mix them together for a certain period of time before completely switching over to another one.

Otherwise, it’s said that this could be a shock to your car’s system, quite similar to how suddenly changing your pet’s food could lead to an upset stomach.

Fortunately, our cars are not that sensitive. As long as you choose a product that’s compatible with your vehicle, you should be fine, and there should be no need to ease your engine through the change by mixing.

Know Your Formula

Take a look at the label on any bottle of lubricant. You’re not likely to see a list of each and every single compound that’s inside it.

With all the additives and chemical refinements, all those names wouldn’t fit. Plus, manufacturers like to keep their formulas a trademark secret, as this gives them an edge over the competition.

This applies to blended formulas, conventional ones, and fully synthetic brands.

You may think that by blending your own, you’re doing a better job. After all, what better way is there to tell exactly how much of each type you’re getting? When you do it yourself, you know what’s going on.

While there’s definitely some truth to that, the same ratio issue will arise here, making your efforts more or less pointless. A greater ratio of full synthetic oil to conventional oil is needed in order to achieve the desired results.

Can You Mix Synthetic Oil With Regular Oil?

So, let’s get to the million dollar question already: Can synthetic oil be mixed with regular oil? And what happens if you mix the two?

The short answer to that question is yes, it’s possible to combine the two. Your engine won’t explode or freeze up if you do.

Hold on for just one moment, though. Before you rush out to buy two different types to mix, wait a second.

We’re going to explain when doing this is a good idea, and when it’s best avoided.

When Should You Mix Synthetic Oil with Regular Oil?

We’re going to have to be totally honest here – there are really only two scenarios where combining two different lubricants should be a viable option to consider:

Low on Oil

The first reason is when you don’t have much of a choice because you’re low on oil.

In these cases, you’re obligated to add whatever is available to hand at the time. This can be a brand that is different from the one you usually use.

The most important thing to do in this situation is to not let your vehicle run out of oil.

Switching Motor Oils Without Flushing

Sometimes, you may want to switch oils without performing an engine flush.

In this case, you’re technically mixing; your new brand oil will mix with the residue of the old.

Note that you shouldn’t do this if your engine is due for a flush. By doing this, you’re risking clogging up the inner mechanics if you leave it dirty and add fresh lubricant.

How Do You Mix Synthetic and Conventional Oil?

So, you’re not considering entirely switching to synthetic oil, but you want to reap the benefits that both have to offer.

If you must mix these two, there are a couple of tips to bear in mind.

You can’t just pick any two random products and combine them without taking the following issues into consideration first. They need to be safely mixed, and you need to know what you’re doing.

Vehicle Compatibility

First and foremost, both brands must be compatible with your vehicle. This is especially true for more modern engines.

Don’t add something that’s been specifically designed for diesel engines into your gasoline-run car.

Similar Additives (If Possible)

We know that it’s not always easy to tell what additives are in a formula. However, you can still try to pick products that have similar properties.

For example, you can pair those that advertise to be detergent-rich together.

The reason for this is because one oil additive can destabilize another if they’re dissimilar.

Identical Weight

The viscosity grade, or weight, must be identical.

Each weight responds differently to temperature extremes and high pressures, so mixing light and heavy lubricants together could compromise engine performance.

There’s no way to mesh the two together without refinement or chemical alteration. Since we’ll assume most of you don’t have a laboratory at home, this won’t be possible.

When Should You Avoid Mixing Synthetic Oil With Regular Oil?

Most of the reasons people have for mixing synthetic oil and conventional oil together are best avoided.

As we’ve touched on earlier, these are:

  • Improving lubricant or engine performance.
  • Transitioning between switching brands.
  • Reducing cost by conserving the synthetic brand.
  • Extending lifespan to delay scheduled changes.

Lastly, if you own an antique automobile, you might seriously want to consider steering clear of mixing the two together.

Older vehicles require more care than newer, better-built models. Engines have really come a long way over the years, and older systems may be somewhat fragile if they’re original. They may also be susceptible to corrosion.


We’ll end this article with the same question that we began with: Can you mix synthetic and conventional oils?

To summarize everything we’ve covered: yes, but only do so if absolutely necessary. That means that if you’re caught out with no other options to use, or you don’t want to flush your engine before switching oils.

As for other common incentives behind doing this, it’s preferable to avoid them. Hopefully, we’ve managed to clear up any misconceptions you may have had.

Engine performance won’t be magically enhanced by splashing synthetic into your conventional. Other benefits, such as improved lifespan, are based more on myth than fact.

If you want the best of both variations, buy a blended brand. Otherwise, stick with the oil you’re already using, or switch brands if you’re unhappy.

From childhood go karting and motocross, to collecting and obsessing over scalextric, matchbox and radio controlled cars, I've always had an obsession with cars. Learning through manuals, books, trial and error, and more knowledgeable family members, I've also enjoyed tinkering with the mechanics and electronics of any vehicles I've owned. Now, over 3 decades later, I've started this site as a place for me to share my knowledge, to teach others how to care for and maintain their vehicles themselves, at home, so they can get the most of their vehicles and save a pretty penny compared to always seeking out professional help.

1 thought on “Can You Mix Synthetic Oil with Regular Oil? Keep Your Engine Safe!”

  1. Alright, my real question is can I put synthetic in my 2001 v6 Camry with 155k on the odometer after it has always had conventional oil in it? changed every 5k.


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