Oil Base Stocks and Additives
Carbon and hydrogen molecules known as hydrocarbons make up the foundation of mineral oils. Different types of hydrocarbon molecules have different characteristics, and some types are more desirable for creating motor oil than others.
The foundations of synthetic oils vary: Some are chemically altered mineral oils, while others are esters or polyalphaolefins.
Let’s separate these molecules into two groups—mineral base oils and synthetic base oils—and discuss their different characteristics.
Mineral Base Stocks
There are three main groups of mineral base oil:
The first are paraffinics, which may be further divided into two subgroups: normal paraffinics and iso-paraffinics.
Normal paraffinics are straight-chain hydrocarbons. Because they are waxy, iso-paraffinics are typically preferred. The latter contain side chains that improve the viscosity index. They also have better oxidation stability, all of which has earned them a reputation as being the best mineral lubricants.
Naphthenics have the characteristic of naphthenes—saturated hydrocarbons with molecules containing at least one closed ring of carbon atoms. Similar to ring compounds like aromatics (but lacking double bonds), naphthenics are considered better than aromatics, but inferior to paraffinics.
Aromatics are unsaturated molecules with one or more benzene rings. Because aromatics are undesirable for motor oils, they’re normally extracted, and only trace amounts are left after refining.
Synthetic Base Stocks
There are several main groups of synthetic base oil used in automotive applications:
Polyalphaolefins (PAOs) are branched-chain, isoparaffinic hydrocarbons known as synthetic hydrocarbons. They’re similar to mineral hydrocarbons, but instead of being extracted like mineral oils, PAOs are manmade using a chemical process. The result is purer, uniformly sized molecules that are completely saturated—they have high oxidative and thermal stability, a high viscosity index, and a very low pour point. They’re also superior in extremely hot or cold temperatures. This highly versatile base stock is great as high-performance motor oil, and superior to mineral oil. The only downsides are a low solubility (which results in poor additive compatibility) and a high price.
Esters like diesters and polyol esters are branched synthetic hydrocarbons that are structurally similar to PAOs. The difference lies in an ester’s hydrocarbon molecules, which contain oxygen in the form of ester linkages. These linkages polarize the molecules, which results in esters having a higher flash point and lower volatility compared with PAOs. The polarity also helps esters “stick” to the engine’s metal surfaces, which gives them additional film strength and lower energy consumption. They’re also great detergents, more environmentally friendly, and they’re very flexible so engineers can build a specific oil for specific applications. Downsides are that esters can affect the elastomer material used in engine seals, and they can react if they come into contact with water. While esters were some of the first synthetic automotive oils, they’ve been surpassed by PAOs due to a PAO’s lower cost and ease of formulation.
Through advanced chemical processes, the new Group III+ base oils (which we’ll discuss shortly) can now be considered a “synthetic.”
As defined by the American Petroleum Institute (API), a base stock is “a lubricant component that is produced by a single manufacturer to the same specifications (independent of feed source or manufacturer’s location); that meets the same manufacturer’s specification; and that is identified by a unique formula, product identification number, or both.”
Base stocks are divided into five categories, or groups: Group I, Group II, Group III, Group IV, and Group V. Through a specific test method, each group has a different makeup of saturates, sulfur percentages, and viscosity index.
Group I: Base stocks that contain less than 90 percent saturates and/or greater than 0.03 percent sulphur, and have a viscosity index greater than or equal to 80 and less than 120.
Group II: Base stocks that contain greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulphur, and have a viscosity index greater than or equal to 80 and less than 120.
Group III: Base stocks that contain greater than or equal to 90 percent saturates and less than or equal to 0.03 percent sulphur, and have a viscosity index greater than or equal to 120.
Group IV: Base stocks are polyalphaolefins (PAO) with no unsaturated hydrocarbons or sulfur. PAOs may be interchanged without additional qualification testing, as long as the interchange PAO meets the original PAO manufacturer’s specs in physical and chemical properties.
The following key properties need to be met in the substituted stock:
1. Kinematic viscosity at 100, 40, and -40 degrees (C)
2. Viscosity index
3. Noack rating (volatility)
4. Pour point
Group V: Base stocks that include all other base stocks not included in Group I, II, III, or IV. Group Vs like esters are typically used for creating oil additives.
Group III+ Base Stocks
While not currently included in the base-stock groups, III+ base stocks bridge the gap between the highly refined mineral Group III base stocks and the expensive, synthetic Group IV base stocks. They provide performance that’s above what normal mineral oils can give, yet they’re more affordable than the Group IV synthetics—and as you can imagine, they’re very popular these days. ExxonMobil’s Visom is an example of a Group III+ base stock.
Oil Additive Packages
Motor oil is made by combining base oil with a complex mixture of up to 15 additives. Generally speaking, there are two types of fluids that make up an additive package: a detergent-inhibitor package, and a viscosity-index improver.
The detergent inhibitor (or DI) package is a mixture of performance additives that’s needed for an oil formulation.
- Anti-foams are non-soluble silicone molecules that help prevent oil from foaming, and also help disperse foam that’s already formed.
- Antioxidants like amines and phenolics help fight oxidation, a reactive process that can create carbon deposits in an engine.
- Detergents are metals and organic chemicals that clean engine deposits and neutralize acid byproducts from combustion. Common metal atoms used include calcium, magnesium, and sodium; common organic products include salicylates, sulfonates, and phenates.
- Diluent oil, or carrier oil, is a mineral oil that helps the solubility of the additives, and ensures that the viscosity is in the correct range for pumping and blending.
- Dispersants suspend and disperse the solid pieces left over from the combustion of fuel. As those pieces would otherwise end up as engine sludge, dispersants are kind of a big deal. Not surprisingly, they’re one of the main components of a DI pack.
- Friction modifiers are molecules that, thanks to their polarity, attach to an engine’s metal surfaces to improve lubricity. They can also improve fuel economy. Organic friction modifiers are typically esters or glycerol mono-oleates (GMO). Inorganic Friction modifiers are molybdenum compounds.
- Pour-point depressants keep the trace amounts of paraffins in oil from growing crystals. PPDs aren’t usually needed in PAO- and ester-based oils, as they don’t contain wax.
- Rust/corrosion inhibitors attach to, and protect, engine metals like iron from acid-, oxygen-, and water-based corrosion.
- Seal conditioners are esters that keep seals pliable. They are especially needed in Group III and Group IV oils, which are known to shrink and harden seals.
- Zinc dialkyldithiophosphate (ZDDP) has been used for years as an affordable anti-wear, extreme-pressure, antioxidant additive. ZDDP is also the most well-known additive, as it is both erroneously vilified for ruining catalytic converters and O2 sensors, and celebrated for keeping flat-tappet engines alive. We’ll discuss ZDDP in more detail in the Additives section.
Viscosity-index improvers, as their name implies, improve a finished oil’s viscosity index. The key to VIIs is their large polymeric molecules: They coil up when cold, which makes little difference to the oil’s viscosity. But when hot, VIIs uncoil and stretch out, which reduces oil thinning—and increases the oil’s viscosity. That improved viscosity index allows manufacturers to create multi-viscosity oils.
Additives, Specialty Oils, and Flat-Tappet Protection
Since the beginning of the automobile and up to the early 1970s, drivers relied on mineral oil to protect their engines. Then came synthetics, with their superior protection qualities (and a few shortcomings, like making engine seals shrink). The oil companies learned from early mistakes, and made synthetics more user friendly. Today, oil engineers can spend their entire careers on developing new additive packages. Why is that important? Read on.
You may be aware that many older engines used flat-tappet cams. And pre-1990s oils had high (1,000-plus ppm) levels of zinc and phosphorus (specifically, zinc dialkyldithiophosphates, or ZDDP), which provided the protection those flat-tappet engines needed.
But in the ’90s, ZDDP levels were reduced in an effort to better protect oxygen sensors and catalytic converters. The auto industry had largely moved to roller cams anyway, so the 800 parts-per-million limit of ZDDP worked fine with newer engines.
However, those ’90s oils’ ZDDP levels were too low to protect flat-tappet engines, and as the tales of engine wear appeared, the aftermarket exploded with ZDDP additives.
To this day there’s a dedicated additive following out there, and some enthusiasts even use diesel-specific oils in an attempt to better protect their valuable engines. But according to the experts we consulted, you shouldn’t use those additives (or diesel oils) in your Corvette.
Why? Oil is a delicate mixture, and adding ZDDP or other substances to your oil can upset that mix. For that reason, in most cases, running additives does more harm (in the form of wear) than good.
If you’re concerned about protecting your Corvette’s flat-tappet engine, or just want a bit more protection for your late-model C4-C7, you have a few options:
1. Use Mobil 1’s 15-50. This heavier-weight oil offers increased ZDDP content, added film strength, and better high-temp protection. And even though it has around 1,200 ppm phosphorus and 1,300 ppm zinc, it’s still API certified, as this viscosity isn’t required to have the lower ZDDP level.
2. Use a “street/’strip” oil like Brad Penn Grade 1. Brad Penn, while not a household name, has a great reputation for producing quality oils. Pennsylvania-grade crude oil is known for high-quality base stocks and great film strengths. Grade 1 adds high ZDDP concentrations that, while not recommended for late-model vehicles, is said to provide outstanding protection for flat-tappet and roller engines alike. (Late-model owners should try Brad Penn’s FSGF synthetic, an API SN-level oil with excellent anti-wear properties and catalytic-converter/oxygen-sensor friendliness.)
3. Use an oil specifically for your engine type, like Driven Racing Oil’s LS30 synthetic for ’97-up LS-powered Corvettes. These high-tech mills have unique issues: lifters ticking during startup, PCV blow-by, roller-cam lobe wear, and lifter and bearing failure. Driven claims that LS30 provides optimum oil flow on startup, which eliminates lifter ticking. It also features a high-temp, high-shear blend that can withstand an LS’s high-temp, high-rpm environment. Its low volatility prevents PCV issues and oil consumption, and the high zinc content protects aggressive roller cams. LS30 can even keep the variable valve-timing system running smoothly in VVT engines like the ’14 Vette’s LT1. 3.
While new-vehicle owners can just take it easy for 500 miles then change the oil, engine builders must break in each new mill carefully. And break-in oils are carefully formulated with the right viscosity to seat the rings, as well as the right anti-wear additives that protect the cam and valvetrain, and minimize damaging metal particles created during break-in.
Flat-tappet-engine builders are typically conscientious about using a proper, high-ZDDP break-in oil like Brad Penn Grade 1 30-weight to protect internal components. However, some roller-motor owners mistakenly believe that their engines don’t need break-in oil. In reality, without a purpose-specific lubricant like Driven BR Break-In Oil, those roller mills will suffer the same particulate contamination—and potential bearing failures—that a flat-tappet engine would. So you ’87-up Corvette owners, take heed: If you’re rebuilding your roller Vette engine, break it in right!
Racing oils are different from street oils, because the engines and operating environments are different.
Modern street engines typically operate in low-rpm, low-load environments, and many are overhead-cam designs—all of which requires fewer anti-wear additives. A large emphasis is placed on emissions, so the engines use EGRs and other emissions equipment, and require additional detergents. And because of ever-increasing drain intervals, additional acid neutralizers are necessary.
Race engines operate in high-rpm, high-temp environments, and many use flat-tappet cams and pushrods—which need more anti-wear additives and friction modifiers. Emissions controls typically aren’t used, so there’s less need for detergents. Racing oils are generally lower in viscosity (though many are still multi-viscosity types, like street oils), but have zinc, phosphorus, and moly levels beneficial for anti-wear protection. However, they’re designed for low-mileage use, with drain intervals usually before 500 miles.
Dexos and the ’14 Stingray
You may have heard about GM’s new oil called dexos1. Developed specifically for GM gasoline engines, dexos1 is the factory-fill oil in vehicles like the Cadillac ATS and the ’14 Stingray.
Some characteristics of the dexos1 synthetic-blend oil:
- Improved viscometric properties, for less friction in the engine and improved fuel economy
- Resists aeration, which enables fuel-saving devices, such as variable valve timing, to work optimally
- Reduces oxidation and deposits, allowing emission systems to operate longer and optimally
- Resists degradation between oil changes, extending the time and mileage interval between oil changes
You can use GM’s dexos1, or you can choose to use one of the GM-licensed dexos1 products found at www.centerforqa.com/gm/dexos1-brands.