Anti-oxidants in engine oils play a big role in keeping your engine safe, cool, and running smoothly. When an engine gets hot, the oil inside can break down and turn into harmful chemicals that thicken the oil and cause wear.
This is where antioxidants step in—they fight the heat, stop damage, and keep the oil fresh for longer. In this guide, you’ll learn how oxidation harms oil, how antioxidants protect your engine, the types used in oils, and how experts test them. Understanding this will help you see why antioxidants are one of the most important protectors inside your engine.
Understanding Oil Oxidation – The Hidden Enemy
Oil oxidation starts when the oil gets too hot. Extreme heat breaks the oil molecules apart and turns them into free radicals. These free radicals are unstable, so they react quickly and start attacking other oil molecules. This creates a chain reaction that keeps getting worse and faster over time.
Oxygen also joins this reaction. When oxygen mixes with oil, it forms hydroperoxides, which are not stable either. They break down into even more free radicals, so the damage continues. Metal parts inside the engine make this process even faster, which means oxidation can spread quickly if not controlled.
If oxidation keeps happening, the oil becomes thicker and harder to flow. This increase in viscosity can slow down engine performance. The process also creates acids that may corrode engine parts and reduce the power of important additives. Over time, sludge and varnish start building up inside the engine, blocking oil paths and trapping heat. This can lead to poor engine protection and long-term damage.
How Antioxidants Protect the Oil
Antioxidants help stop oil oxidation, and they work in two main ways. Each type of antioxidant fights the problem at a different stage, which is why good oils often use more than one type for stronger protection.
- Primary antioxidants are also called chain breakers. They stop the oxidation reaction by giving a hydrogen atom to the free radical. When they do this, the free radical becomes stable and harmless. The antioxidant is used up in the process, but it turns into a safe and inactive compound, which means it no longer causes damage.
- Secondary antioxidants work a little differently. They attack hydroperoxides, which form early during oxidation. These hydroperoxides can break down and create even more free radicals, so secondary antioxidants break them apart before that happens. By removing hydroperoxides, they stop new free radicals from forming and slow down the whole oxidation process.
Types of Antioxidants and How They Work
Antioxidants are chemicals added to engine oil to stop oxidation. There are different kinds, and each one has its own strengths. Using more than one type together makes the oil much stronger and lasts longer.
Phenolic Antioxidants
Phenolic antioxidants are the most common primary antioxidants used in engine oils. Examples include BHT and other hindered phenols. They work well with many base oils and other additives, and they stay stable even under heat.
These antioxidants are great for engines that run at moderate temperatures, usually up to 150°C. Their structure makes it easy for them to donate hydrogen and stop free radicals from spreading. This helps keep the oil healthy for a long time.
Aminic Antioxidants
Aminic antioxidants are another strong type. They include diphenylamines and alkylated diphenylamines. These work very well in engines that face high temperatures, even above 200°C.
They are often used for heavy-duty engines and high-performance vehicles. One thing to know is that they may darken the oil color over time, which does not harm performance but may matter in some applications.
Secondary Antioxidants
Secondary antioxidants include phosphites and thioesters. They do not fight free radicals directly. Instead, they break down hydroperoxides, which are early byproducts of oxidation. If hydroperoxides are not removed, they can create more free radicals later.
- Phosphites (like tris(nonylphenyl) phosphite) break down hydroperoxides and stay stable under heat.
- Thioesters (like dilauryl thiodipropionate) are great for long-term protection and help block metal-related oxidation.
Why Blending Antioxidants Works Best
The strongest oil protection comes when different antioxidants work together. Primary antioxidants stop free radicals, while secondary antioxidants remove hydroperoxides before they turn into more radicals. When both are used as a team, they protect engine oil much longer than either could alone.
The amount of each antioxidant depends on the type of oil, temperature, and how hard the engine works. Blending them correctly creates the best balance and the longest oil life.
How Antioxidants Help Stop Oxidation in Lubricants
Antioxidants are chemicals that help slow down oxidation in engine oil. Because of this, they are also known as oxidation inhibitors. When base oil is refined, most of its natural antioxidants are removed. That means new antioxidants must be added later to protect the finished lubricant. Antioxidants do not stop oxidation completely, but they greatly reduce how fast it happens.
Base oils originally contain some natural antioxidants like polycyclic aromatics, sulphur, and nitrogen compounds, but these are usually lost during refining. When oil begins to oxidize inside the engine, it forms acids and peroxides. To control this, finished lubricants are often mixed with antioxidants such as hindered phenols and ZDDP (zinc dialkyldithiophosphate). These help slow the formation of acids and protect the oil.
There are different types of antioxidants, including amines, phenols, and sometimes ZDDP. ZDDP also works as an anti-wear additive, so it protects moving metal parts while fighting oxidation. In many diesel and gasoline oils in Europe and North America, antioxidants make up about 3–7% of the additive package.
Antioxidants do not work alone. They often work together with detergents, which help prevent corrosion. When acids and moisture enter the engine, detergents reduce the damage they cause. Alkylphenols are sometimes used to make detergents, and they also provide some antioxidant strength.
Other additives like extreme-pressure agents that contain sulphur or phosphorus may also reduce oxidation. However, they break down at lower temperatures, so they are not promoted as strong antioxidants.
Testing Methods for Detecting Antioxidants in Lubricants
Now that we know how antioxidants work, the next step is learning how to check if they are still in the finished oil. The industry uses different tests to measure oxidation and understand how much antioxidant protection may be left in a lubricant.
There are two general ways to check antioxidants:
- Measure how fast oxidation is happening: These tests don’t show the exact amount of antioxidants remaining, but they help us understand how much the oil has already aged.
- Direct tests that show which antioxidants are present and how much is left: These tests give clearer results, but they are less commonly used.
Some common industry tests that measure oxidation rate include:
- RPVOT (Rotating Pressure Vessel Oxidation Test)
- FTIR Oxidation Test
- Viscosity Check
- TOST (Turbine Oil Oxidation Stability Test)
None of these tests tell the exact antioxidant level, but they help us see how fast oxidation is happening.
RPVOT Test
The RPVOT test has been used for many years. In this test, an oil sample is placed in a sealed container filled with pressurized oxygen. The container is spun at a high speed inside a heated bath to speed up oxidation.
As oxidation happens, the pressure inside the vessel drops. The result is measured in minutes. If the value drops below 25% of the original number, it means the oil is near the end of its useful life. However, even high values like 800 minutes do not tell us how long the oil will last in real-world use. This is why RPVOT does not measure the true amount of antioxidants left.
FTIR Oxidation Test
FTIR (Fourier Transform Infrared Spectroscopy) detects oxidation by reading a “fingerprint” pattern. Oxidation shows up as a special peak between 1600–1800 cm⁻¹. The results are usually compared with new oil samples.
This method is mostly used for engine oils and only shows that oxidation has already happened—it does not tell how much antioxidant remains.
Viscosity Check
In the past, viscosity was sometimes used to check oxidation. But we now know viscosity only increases after oxidation has already taken place, usually because of sludge or varnish. Since viscosity can change for many other reasons too, it cannot accurately show remaining antioxidant levels.
TOST Test
The TOST test, created in 1943, measures how long it takes for oil to form acids under harsh conditions. The oil is heated to 95°C (203°F), mixed with water, and exposed to air and metal catalysts. The time it takes to reach a certain acid level shows how well the oil can resist oxidation.
However, this test still does not tell the exact remaining antioxidant amount. It is used mainly for steam turbine oils, not hydraulic or gear oils.
Final Takeaways
Antioxidants are one of the most important—but often overlooked—parts of engine oil. They stop the oil from breaking down, helping it last longer and keeping your engine safe and reliable. Knowing how antioxidants work and the different types available makes it easier to choose the right oil for your engine.
Fubex uses carefully designed antioxidant systems in its oil additives to give the best protection against oxidation. Our team helps customers pick the right solution for their engine based on how it’s used and the conditions it faces.
FAQs
Q1: Why are antioxidants used in engine oil?
Antioxidants help engine oil last longer by slowing down oxidation. This means the oil can handle higher temperatures without breaking down, giving better protection and longer life to your engine.
Q2: Why do we add antioxidants to engine oil?
Antioxidants are added to engine oil to make it more resistant to oxidation and last longer. They help slow down oil breakdown and work together with other additives to keep the lubricant protecting your engine effectively.
Editor-at-Large
A passionate writer in the lubricant industry, Awais Iqbal has been covering oils, greases, and industrial fluids since the start of his career. At 25, he’s already written for blogs, catalogs, and brand guides across the UAE. Awais’s insights help companies connect with their audience, and his clear, helpful writing style is trusted by brands in the region.
