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Motor Oil Additives and Modifiers Explained


Oil splash coming from a bottle of motor oil

In addition to the base oil, almost all motor oils contain many additives to enhance the oil's performance as an engine lubricant.

Just to be clear, the additives we'll be talking about on this page are those that the motor oil manufacturer adds to the base oil, not anything that consumers might add to their cars' oils. The additives discussed on this page are already in the oil when you buy it.

The collection of additives added to a particular motor oil are often called its additive package. Different manufacturers use different packages, most of which are proprietary and some of which are secret. In many cases, it's the additive packages, more so than the base oil, that determine the price points of oils with a manufacturer's lines.

Manufacturers also vary their additive packages based on the primary intended purpose of the oil. Oils intended for diesel engines, for example, tend to have much higher levels of ZDDP than oils intended for gasoline engines because ZDDP is rough on gasoline-powered cars' catalytic converters. It's vital to use an oil made for (or at least approved for) the type of engine in your vehicle.

Let's take a look at some of the more common motor oil additives and modifiers.

Types of Motor Oil Additives

Viscosity Improvers and Modifiers

Pour Point Depressants

Pour point depressants lower the temperature at which a motor oil can easily be pumped through an engine. Because even momentary oil starvation can cause catastrophic damage to an engine, it's critical that the oil start flowing immediately when the engine is started, no matter how cold it is outside.

Many synthetic oils (including many Group III synthetics) don't contain any pour point depressants because synthetic base oils tend to flow well at low temperatures without the need for any additives. This is because unlike Group I and Group II petroleum-based oils, synthetics contain little or no wax. It's the wax contaminants in conventional oils that crystallize and cause the oil to thicken at cold temperatures.

Viscosity Index Improvers

The viscosity index, or VI,refers to an oil's viscosity stability over the working temperature range of an engine. VI improvers are polymers added to an oil to reduce its tendency to lose viscosity at high temperatures. Polyalkylmethyl acrylates (PAMA) and Olefin copolymers (OCP's) are two of the viscosity-improving polymers commonly used in engine oils.

Viscosity index improvers work by a shape change that occurs in the polymer molecules as their temperature increases. If you picture the molecule as a coiled spring, imagine it uncoiling when its temperature increases and you'll get the basic idea. As the VI improver's molecules "stretch out," for lack of a better way to put it, they causes the oil to thicken.

The problem with viscosity-improving polymers is that they are susceptible to "shear," or molecular damage, at high temperatures and stresses. The bigger the molecule, the more easily it will shear. This is one of the reasons why motor oil needs to be changed. The longer the oil is in use, the more of its VI improvers will have been sheared and rendered ineffective.

Because synthetic base oils have inherently better viscosity stability, they are able to use VI improvers with much smaller molecules. These smaller molecules are less susceptible to shear damage and therefore last longer. That's one of the reasons why longer oil-change intervals are possible when using synthetic oils.

Friction Modifiers

Friction modifiers reduce the friction between the engine's moving parts by preventing those parts from coming into direct contact. They help the oil form protective layers between the parts so they touch the oil rather than each other. Some of the more common materials used as friction modifiers include natural or synthetic fatty acids, polymers, graphite, molybdenum, and nanoparticles.

Like VI improvers, most friction modifiers break down and lose their effectiveness over time. That's another reason why oil has to be changed.

Anti-Wear Additives

Anti-wear additives work a bit differently from friction modifiers. Most of them only become reactive under conditions of extreme temperature or pressure. They react with the surfaces of the metal and form a sacrificial layer on the parts that protect the parts themselves from wear.

One of the oldest and most-popular anti-wear additives is zinc dialkyldithiophosphate (ZDDP). At high temperature and pressure, ZDDP breaks down and forms a phosphate coating on the metal surfaces that protects them from wear. It works so well that it's the main ingredient in many consumer oil supplements.

The problem is that those same phosphates that protect your engine are murder on a gasoline-powered vehicle's catalytic converter. Even dually-rated diesel engine oils that are also approved for gasoline engines usually contain more ZDDP than I'd personally want in an oil for long-term use in a gasoline engine with a CAT. ZDDP is also toxic to aquatic wildlife, and some environmentalists believe that the collective effect of oil leaked from engines washing from roads into bodies of water can harm fish populations.

My advice is not to add ZDDP nor any other oil additives to your engine's oil. Buy high-quality oil to begin with, and you won't need them. If you have a diesel vehicle, use a high-quality diesel engine oil like Shell Rotella or Mobil Delvac. If you drive a gasoline-powered vehicle, use a high-quality oil made for gasoline engines. Some of the better oils for gasoline engines include Castrol Edge Titanium, Pennzoil Ultra Platinum, Mobil 1, and AmazonBasics Synthetic; but once you get to the top shelf, most of the major manufacturers' flagship oils are excellent.

Other chemicals used as anti-wear additives include boron, chlorine, phosphorous, sulfur, zinc, and others. As is the case with ZDDP, these chemicals are added in precise dosages by the oil's manufacturer. Adding additional consumer additives is unlikely to provide any meaningful improvement in protection for average cars driven by average drivers, and may in fact cause more harm than good. My advice, again, is to buy high-quality oil to begin with, change it regularly, and skip the oil supplements.

Anti-Corrosion and Anti-Oxidation Additives

Hydrocarbons naturally react with oxygen, which causes them to break down. That's one of the reasons why oil has to be changed according to time as well as mileage. Oil does "go bad" after a while, and oxidation is one of the reasons. Even oil in sealed jugs is considered to have a shelf life of only about five years, and oil in opened jugs a shelf life of only one year once the seal has been broken.

Anti-oxidation additives slow, but don't entirely stop, the damage due to oxidation. Primary antioxidants work by neutralizing free radicals in the oil. Secondary antioxidants work by breaking down hydroperoxides and peroxides in the oil. Free radicals and peroxides are destructive to the oil because they form oxygenated hydrocarbons and acids that can harm the engine. Neutralizing or decomposing them helps protect both your oil and your car.

Anti-corrosion additives work by coating the engine's parts so they don't come in contact with corrosive substances, and by neutralizing the acids that are formed by the combustion process and by the effects of reactive chemicals in the oil. An oil's ability to neutralize acid is called its Total Base Number (TBN). The higher the TBN, the better the oil's ability to neutralize acid. (Base, in this case, refers to chemicals with high pH, or the opposite of acids.)

Detergents and Dispersants

Detergents are additives added to base oil to prevent and, in better oils, help remove deposits like varnish and sludge. Some detergents work by breaking down oxygenated hydrocarbons that tend to form sludge, and other detergents attempt to break down existing sludge deposits so they can be carried away by the oil.

Diesel engine oils contain much more powerful detergent packages than gasoline-engine oils. Some people like to use diesel-engine oils that are also rated for gasoline engines because they believe they'll "keep the engine cleaner." That's probably true; but as mentioned earlier, diesel engine oils usually have higher ZDDP levels that might shorten the life of your catalytic converter.

Dispersants are additives that help keep contaminants in suspension in the oil. The bigger particulate contaminants can then be filtered out by the oil filter, and the smaller ones held in suspension until the oil is changed. Some synthetic oils have special dispersants designed to keep metals in suspension because synthetic base oils don't have very good metal-carrying capabilities. Synthetic oils made for aviation piston engines also need metal-dispersing additives because piston-engine aircraft typically burn 100LL leaded gasoline.

Anti-Foaming Additives

When you add detergent to the water in a washing machine, it foams. The same thing happens when you add detergent to motor oil in an engine. But unlike foam in your washing machine, foam in your engine oil can cause serious problems. Anti-foaming agents help reduce the tendency of the oil and its detergent additives to foam.

Foam in oil is basically air bubbles, and air bubbles can't protect the moving parts of an engine. Air can't form a protective film between moving parts that prevent them from touching each other; it can't protect metal parts from corrosion; it can't help carry heat away from the engine; it can't neutralize acids; and so forth. That's why motor oil needs foaming inhibitors.

Foam is also one reason why you shouldn't overfill the oil. When the oil level is too high, the moving parts of the engine can whip the oil into foamy bubbles that can't protect the engine's moving parts.

Common Additives in High-Mileage Motor Oils

Seal Swellers and Conditioners

Almost all high-mileage motor oils except diester-based synthetics (which tend to swell seals without additives) contain seal swellers, and most contain additional conditioners that lubricate the seals and attempt to restore some of their resiliency. Sometimes they work, and sometimes they don't. They're probably worth a try on an old car that's leaking oil when replacing the seals isn't an option.

Cylinder and Ring Additives

Some high-mileage oils contain additives (usually metals like silver, copper, and lead) that attempt to fill fine scratches in the cylinder walls to reduce blow-by and restore compression. Again, sometimes they work, and sometimes they don't. If a top overhaul is out of the question, they're worth a try.

Additional Viscosity Index Modifiers

Many older engines burn oil because of worn rings. Some high-mileage oils attempt to reduce this problem by adding additional VI modifiers to thicken the oil at operating temperature. If the only problem you're having is oil burning, however, you may want to try switching to an oil whose upper viscosity (the number without the "W") is at the high end of the manufacturer's recommended viscosity range, rather than a high-mileage oil.

For example, if either 5W-20 or 5W-30 are allowed, and the only problem you're having is that the engine is burning oil using 5W-20, I suggest you try 5W-30 before trying a high-mileage oil. You don't want to swell the seals, which is one thing high-mileage oils are designed to do, if they're not leaking. Using a higher-viscosity oil (but one that is still within the car manufacturer's approved range) may (or may not) reduce the oil-burning problem without swelling the seals.

Another thing you can try if your gasoline-powered car is burning oil, but is having no other problems, is treating a tank or two of gas with Techron or another combustion chamber cleaner containing a high percentage of PEA (polyether amine). Built-up carbon crud between the rings and the cylinders can cause oil burning. PEA helps break down those deposits. Of course, if carbon deposits aren't the reason for the oil burning, then the Techron won't solve the problem. But it's easy to try, won't hurt, and might help.