There’s a lot of complex chemistry behind engine oil for something so seemingly simple. Different types and brands use different base oils and have special additives. However, the most important factor in any engine oil is viscosity.
Most automakers provide you with basic recommendations on the viscosity of engine oil you should use in your car. This is based on data-driven engineering for the type of engine and the type of engine material.
Within each brand, there are various viscosity ratings to meet most modern automaker recommendations. Synthetic, synthetic blends, and traditional engine oils divide the lines yet further, leaving you to wonder if it’s possible or even a good idea to mix different types of engine oil.
The good news is that the American Petroleum Institute requires all engine oils of the same viscosity rating to be compatible with each other. This means you can mix them, and they won’t damage your engine. However, they likely won’t have the same additives and might not even have the same base oil, which can affect how the oil performs in your engine in the long term.
To understand why it might not be a good idea to mix engine oils in all but an emergency situation, we’re going to have to take a little bit of a deep dive into this murky fluid to explore its differences. This starts with asking and answering a few common questions.
What Do Oil Letters and Numbers Mean?
The numbers and letters on a quart of engine oil indicate its “Weight,” which is the technical term for viscosity. This is essentially the oil’s resistance to flow. The lower the number, the less resistance the oil has to flow.
The first number before the W (Weight) tells you the oil’s inherent temperature in cold temperatures. With a quart of 10W-30, it has a viscosity of 10 at cold temperatures. The second number tells you the oil’s viscosity when the engine is hot.
Do Automakers Recommend Specific Oil Brands?
Some automakers are partnered with oil companies and recommend their oil. However, unless you have an exotic or high-performance engine, the specific brand doesn’t matter. What matters is that you’re always using the same viscosity of oil.
What Happens If I Don’t Use the Recommended Type of Oil?
Using an oil with a different viscosity than the manufacturer recommends will affect the lubrication of the moving parts inside your engine. This can lead to excess friction and heat, damaging various engine components. It can sometimes be so severe that the engine overheats, a piston seizes, a cylinder cracks, or the cylinder head fractures from overheating.
What Happens If I Mix Different Weights of Oil?
Mixing oil weights changes the viscosity of the engine oil, which can affect engine performance in many ways. Depending on the engine type and the driving conditions’ startup temperature, you could even damage some of your engine’s sensitive moving parts.
1. Hard Start-Up
If you mix two different oil weights and give it a thicker viscosity, the engine might have difficulty turning over on days when the weather is cold. The engine might crank and sputter, trying to start as the oil pump struggles to pick up enough oil to keep the pistons and cylinder walls properly lubricated.
2. Running Hot
If your mixed-weight oil has too high of viscosity when warm, the overly loose oil can cause excess friction in the cylinders. You’ll notice the temperature gauge on the dash running hotter than usual. On long drives or in hot weather, you might even get an engine coolant temperature warning light.
3. A Blown Head Gasket
A hot-running engine with the wrong oil viscosity can eventually damage the head gasket. This creates a leak that can allow engine oil and coolant to mix.
4. A Failing Oil Pump
If your mixed oil has a thicker viscosity, the oil pump will strain to pick up enough oil to lubricate the engine’s moving parts properly. If you drive like this long enough, the pump can completely fail. This is an expensive repair and can render the car undrivable or worse.
5. Damaged Cylinder Walls
Anytime you use the wrong viscosity oil for your engine, you risk excess friction contact between the pistons and the cylinder walls. Left long enough, you could start to get scoring, scratches, and deformations in the cylinder walls. This can affect performance as well as make it possible for
Can Mixing Different Oil Brands Damage My Engine
If you mix two different oil brands with the same viscosity rating recommended for your engine, the initial effects will be negligible.
In the past, different oil brands used different additives and detergents that could interact and change the oil. There are famous tales about mixing Pennzoil and Valvoline, causing failures in oil pan gaskets. You’ll even find people on internet forums who still pound the table about the evils of mixing different oil brands.
Though today the American Petroleum Institute mandates that all conventional motor oils be compatible with one another.
So, if your car is designed to run on 10W-30 conventional motor oil, and you need to top it up with a different brand, there shouldn’t be an appreciable difference in the short term.
The Long-Term Effects of Mixing Different Brands of Engine Oil
The concern with mixing two different engine oil brands is that each brand uses different base oils and additive packs that don’t always work together.
1. Reduced Mileage Performance
If you’re using one brand/type of motor oil that has special additives for a high mileage vehicle or added wear inhibitors, and you add a different brand of oil, you won’t get the full benefits of the original motor oil.
If one brand uses different base oils that are rated to last up to 5,000 miles, and you add oil from a different brand that is only rated for 3,000 miles, you can’t reasonably expect to get the original 5,000 miles of life out of the original oil.
2. Increased Risk of Oil Sludge
Any mixed lower-mileage oil will oxidize and break down prematurely, increasing the risk of oil sludge. This can also cause potential friction damage on the engine’s moving parts and cylinder walls.
3. Faulty Oil Pump
If oil sludge develops or excess additives are suspended in the oil, it can make it hard for the oil pump to pick it up. Especially in cold conditions. Over time, this could stress the pump, causing it to fail prematurely.
4. Contaminants in the Exhaust System
There’s the potential for the additive imbalance to blow through and contaminate the oil. This can also leave debris that falls out into the exhaust system. If the catalytic converter is already a little dirty, this could worsen the clog.
5. Leaks in Oil Pan Gasket
It’s possible for additives and detergents from one brand of oil to mix adversely with the additives and detergents from another brand. Given enough time, this could affect the oil pan gasket, leading to minor oil leaks and drops of oil under the car.
So, even though the API now mandates that all engine oils must be compatible with each other, it’s still wise to stick with one brand through the life of the oil change.
What Happens If I Mix Synthetic and Conventional Engine Oil?
While mixing conventional and synthetic engine oil won’t immediately damage your engine, it can affect the additive performance of the synthetic oil. Pure synthetic and synthetic blend engine oils have different additives in them.
Conventional motor oil will impede the synthetic oil’s additive performance. Synthetic oil is also less likely to oxidize and develop oil sludge, though its additives also tend to fall out of suspension if the car sits too long.
Synthetic oil also tends to perform better at high temperatures. This will also be hampered by adding conventional oil to an engine that’s been running on synthetic.
What Happens If I Mix Full Synthetic with Synthetic Blend Oil?
Adding a little full synthetic oil to an engine that’s running on a synthetic blend won’t damage the engine or vice versa. However, synthetic oils tend to have more high-performance additives, and they can vary slightly from one brand to the next.
Full synthetic uses base oil, powder additives, and a special type of carrier oil that ensures an even distribution of the additives throughout the base oil. Synthetic blend engine oil is a mixture of conventional motor oil and synthetic base stock. This gives you better lubricating performance and a longer lifespan than if you were just using conventional motor oil.
Each brand of synthetic engine oil has slightly different high-performance fluids and additives. If you mix different synthetic oil brands, you won’t get the maximum benefits of either type. So, while it won’t hurt your engine, the wise move is to stick to one brand of full or blended synthetic oil.
What Happens If I Put Diesel Engine Oil in My Gasoline Engine?
Suppose you can find diesel engine oil with a viscosity compatible with your gasoline engine, and you need to top up your oil reservoir. In that case, it won’t immediately cause any damage to the engine. However, the wise move is to get an oil change as soon as it’s convenient.
The additives in diesel engine oil can pass through and clog the catalytic converter and other parts of the exhaust system over an extended period of time. So, using conventional motor oil in your gasoline engine is best whenever possible.
What Happens If I Put Conventional Motor Oil in a Diesel Engine?
Using conventional motor oil for a gasoline engine in a diesel engine can cause excess wear and can damage a wide range of engine components. Diesel engine oil has a higher volume of anti-wear additives and detergents to help performance and manage residue in the engine. Using engine oil that isn’t formulated for a diesel engine then leads to increased wear in the valve train, rings / liners, and other high-contact areas in the engine.
Diesel engine oil is also formulated to flow easily in cold temperatures, making it easier for the oil pump to pick it up. If you use conventional motor oil for a gasoline engine, the diesel engine might have a hard time starting in cold temperatures and will be moving through the first strokes with minimal lubrication.
Frequently Asked Questions
Do Different Oil Brands Use Different Additives?
Different brands use different additives in different proportions. This is especially true for full synthetic oils and some synthetic blends. When you start mixing brands, you don’t get the full effects of either type of additives. So, if you do have to add a different brand in a pinch, it won’t hurt your engine. However, you should probably get an oil change as soon as possible. Especially if the oil you added has a lower mileage rating.
Can I Switch Brands for an Oil Change
A standard oil change is designed to drain away the vast majority of the old engine oil. While there will be a little residual old oil in the engine, the new brand you put in will dominate the oil’s inherent characteristics. In the past, this was a bad idea, but today, the API mandates that all oils be compatible with each other.
Can I Put Modern-Day Oil in My Vintage Car?
The API has ratings for various oils. Not all modern-day motor oils are reverse compatible with what the API calls “Obsolete” cars, which might apply to vintage engines. If you want to use modern engine oil in a vintage engine, your best bet is to go with an API oil with an SJ or SL rating.
The American Petroleum Institute mandates that all oil brands need to be compatible with one another. However, that doesn’t mean mixing them wantonly is a good idea. Each brand uses its own base oils and unique additive packs to give the various oils in their line special characteristics. This is especially true for diesel engine oils and synthetic oils, which tend to have a higher concentration of additives per volume than conventional motor oil.
If you need to add oil in a pinch, but you can’t find the brand your engine is currently running on, it’s okay to use another brand to top it up. It’s certainly better than running the engine on low oil.
The key is to ensure you are using exactly the same oil viscosity. If your engine is supposed to run on 10W-30, then make sure you add 10W-30. Otherwise, the change in viscosity might affect engine performance to the point that it could cause engine damage.
You also have to be mindful that you won’t benefit from the original additives. The wise move is to get an oil change sooner rather than later. This will ensure your engine gets full mileage protection and additives that can only come from running on a single type of engine oil.
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.