Select Synthetics

Motor oil quality has changed dramatically in the past 50 years or so, and new demands on lubricants in modern engine designs call for oils that meet very stringent requirements. Variations in oil’s ability to meet the requirements determine which service classification rating and viscosity grade it receives.

Service classifications are determined by the American Petroleum Institute (API), while oil viscosity grades are determined by the Society of Automotive Engineers (SAE). These two organizations have set industry standards for motor oils for close to 100 years.

Motor Oil is classified by two systems. One system determines the oil's viscosity rating (the SAE grade), and the other (the API classification) determines its performance level and appropriate applications. Both of these designations are displayed on the API Service Classification Symbol, also known as the API "Donut".

The API Service Symbol "Donut" is divided into three parts:

1. The top half describes the oil's performance level (for gasoline and/or diesel engines).
2. The center identifies the oil's viscosity.
3. The bottom half tells whether the oil has demonstrated energy-conserving properties in a standard test in comparison to a reference oil, or if an oil meets SN PLUS or CI-4 PLUS requirements.

“API’s Engine Oil Licensing and Certification System (EOLCS) is a voluntary licensing and certification program that authorizes engine oil marketers that meet specified requirements to use the API Engine Oil Quality Marks. Launched in 1993, API’s Engine Oil Program is a cooperative effort between the oil and additive industries and vehicle and engine manufacturers Ford, General Motors, and Fiat Chrysler and those represented by the Japan Automobile Manufacturers Association and the Truck and Engine Manufacturers Association. The performance requirements and test methods are established by vehicle and engine manufacturers and technical societies and trade associations such as (ASTM), (SAE), and the American Chemistry Council (ACC).

The Engine Oil Program is backed by a monitoring and enforcement program that ensures licensees adhere to program requirements. This includes running physical, chemical, and performance tests on licensed engine oils and verifying that the API-registered Marks are properly displayed on containers and convey accurate information to consumers. About half of the program’s licensees are based in the United States, and the other half are spread around the globe.” api.org

In other words, the Licensee pays a Licensing Fee to API which then entitles them to use the coveted API Service Symbol "Donut" on the back label of their Motor Oil as well as the API Certification Mark, also known as the "Starburst" on the front label. (see below) The API 'Starburst' Certification Mark is designed to identify engine oils recommended for a specific application (such as "FOR GASOLINE ENGINES").

The API "Shield" (see below) is a certification mark introduced by the American Petroleum Institute (API) to identify engine oils meeting the strict ILSAC GF-6B standard, which requires superior fuel economy and engine protection, specifically for 0W-16 viscosity grades. It is visually distinct from the older "Starburst" symbol.

An API license indicates that a specific motor oil formulation has passed the minimum performance standards as defined by a series of laboratory bench, physical, chemical and engine tests.

These tests and minimum performance standards are determined by the API Lubricants Committee to address specific areas such as engine wear, deposits, fuel economy and emissions. The committee is comprised of representatives from automobile, oil and additive companies.

Additive companies develop licensed formulas that they offer to oil companies to re-license. It is inexpensive to re-license one of these formulas, and the majority of oil companies choose to do this to avoid the costs associated with testing their own formulas. This lowers the value of such engine oils because the same chemistry is being sold under many different brand names.

While the licensed motor oil must meet the established minimum for performance under a specified API Category, other than this minimum performance requirement, there is no differentiation in any product that exceeds the minimum.

In other words, there are many cheap, low quality, inexpensive conventional oils that meet the API minimum performance standards and display the "donut" and "starburst" quality marks on their packaging. But does this mean that their performance level is on par with the premium quality 100% synthetic oil offerings, such as AMSOIL's top-tier motor oils for instance? Absolutely not! They’re in a totally different performance category. Unfortunately, API doesn't make a distinction between different performance levels.

Additionally, according to an API survey of more than 1,800 oil samples purchased from bulk dispenser tanks in quick lube shops across the U.S., nearly 20% (one out of five) were mislabeled and failed to meet API standards. Either the viscosity was incorrect or the additive package failed to meet the performance requirements.

The American Petroleum Institute (API) developed a classification system to identify oils formulated to meet the operating requirements of various engines. The API system has two general categories: “S” categories and “C” categories. (see images below)

The “S” (Service / Spark ignition) classifications are for oils designed for gasoline-engine service (cars, vans, and light trucks with gasoline-engines).

When oil passes a series of tests in specific engines (API Sequence tests), it can be sold bearing the applicable API Service or “S” classification. The classifications progress alphabetically as the level of lubricant performance increases. Each classification replaces those before it.

Oils meeting the latest API classification, API SQ, may be used in any engine, including older engines where earlier category oils were recommended, unless the engine manufacturer specifies a "non detergent" oil. (SA and SB are non-detergent oils and are not recommended for use in an engine unless specified by the manufacturer.)

API SQ motor oils are designed to provide improved high temperature deposit protection for pistons and turbochargers, improved oxidation resistance and deposit protection, more stringent sludge control, improved fuel economy, emission control system compatibility, seal compatibility, and protection of engines operating on ethanol-containing fuels up to E85.

Not everyone is in agreement with backwards compatibility, and in addition, there are special situations, such as "performance" engines or fully race built engines, where the engine protection requirements are above and beyond API/ILSAC requirements. Because of this, there are specialty motor oils out in the market place with higher than API allowed phosphorus levels (such as AMSOIL’s Z-Rod motor oils as well as their Racing motor oils).

All the current gasoline categories have placed limitations on the phosphorus content for certain SAE viscosity grades due to the chemical poisoning that phosphorus has on catalytic converters. Phosphorus is a key anti-wear component in motor oil and is usually found in motor oil in the form of zinc dithiophosphates (ZDDP) additives.

Each new API category has placed successively lower phosphorus and zinc limits, and thus has created a controversial issue of obsolescent motor oils needed for older engines, especially engines with sliding (flat/cleave) tappets. Most engines built before 1985 have the flat/cleave bearing style systems, which are sensitive to reduced levels of ZDDP additives.

SL (for 2004 and older gasoline-engines), SM (for 2010 and older gasoline-engines), SN and SN Plus (for 2020 and older gasoline-engines), SP (for 2025 and older gasoline-engines), and SQ (for today's modern gasoline-engines) are the currently recognized API classes; previous classifications are now obsolete.

The “C” (Commercial / Compression Ignition) classifications are for oils designed for diesel-engine commercial service (heavy-duty trucks and other vehicles with diesel-engines). The list of past and current C class diesel oils include CA, CB, CC, CD, CD-II, CE, CF, CF-2, CF-4, CG-4, CH-4, CI-4, CI-4 PLUS, CJ-4, CK-4, and FA-4.

Not all C classifications supersede one another. These classification systems aim to help motorists choose the right diesel motor oil for their specific applications. The choice depends on the specific diesel engine, the outdoor temperature and the type of driving the engine must withstand.

Therefore, the currently recognized classifications for modern diesel oils are FA-4, CK-4, CH-4, CI-4, and CJ-4. In addition, API created a separated CI-4 PLUS designation in conjunction with CJ-4 and CI-4 for oils that meet certain extra requirements, and this marking is located in the lower portion of the API Service Symbol "Donut".

CH-4 oils, introduced in 1998, are for high-speed, four-stroke engines designed to meet 1998 exhaust emission standards. CH-4 oils are specifically compounded for use with diesel fuels ranging in sulfur content up to 0.5% weight. They can be used in place of CD, CE, CF-4, and CG-4 oils.

CI-4 oils, introduced in 2002, are for high-speed, four-stroke engines designed to meet 2004 exhaust emission standards implemented in 2002. CI-4 oils are formulated to sustain engine durability where exhaust gas recirculation (EGR) is used and are intended for use with diesel fuels ranging in sulfur content up to 0.5% weight. They can be used in place of CD, CE, CF-4, CG-4, and CH-4 oils. Some CI-4 oils may also qualify for the CI-4 PLUS designation.

CJ-4 oils are for high-speed four-stroke cycle diesel engines designed to meet 2010 model year on-highway and Tier 4 non-road exhaust emission standards as well as for previous model year diesel engines. They are formulated for use in all applications with diesel fuels ranging in sulfur content up to 500 ppm (0.05% by weight). However, the use of these oils with greater than 15 ppm (0.0015% by weight) sulfur fuel may impact exhaust after-treatment system durability and/or drain interval.

CJ-4 oils are especially effective at sustaining emission control system durability where particulate filters and other advanced after-treatment systems are used. Optimum protection is provided for control of catalyst poisoning, particulate filter blocking, engine wear, piston deposits, low- and high-temperature stability, soot handling properties, oxidative thickening, foaming, and viscosity loss due to shear. They exceed the performance criteria of API CI-4 with CI 4 PLUS, CI-4, CH-4, CG-4 and CF-4 and can effectively lubricate engines calling for those API Service Categories.

The latest API "C" category for heavy-duty diesel engine oil is API CK-4, introduced in 2016 for 2017 and later model-year engines. It offers superior protection against oxidation, shear, and engine wear compared to previous standards and is backward compatible with CJ-4, CI-4, and CH-4. API FA-4 was introduced alongside CK-4, this category is designed for fuel efficiency in newer (2017+) on-road engines but is not widely backward compatible. (see images below)

Key Details About Modern Diesel Categories:

• API CK-4: The current standard, designed for high-speed four-stroke engines utilizing low-sulfur diesel, suitable for on-road and Tier 4 off-road applications.
• API FA-4: Introduced alongside CK-4, this category is designed for fuel efficiency in newer (2017+) on-road engines but is not widely backward compatible.
• Upcoming Change: The industry is moving toward API CL-4 (Proposed Category 12 - PC-12) to meet upcoming 2027 emission standards, which is expected to be finalized around 2027.

Note: For European heavy-duty engines, ACEA standards are used instead of API.

These designations apply only to motor oils intended for gasoline engines (cars, vans, and light trucks). (see image below)

Widespread use of “Resource Conserving” or “Energy Conserving” motor oils may result in an overall savings of fuel in the individual vehicle, and vehicle fleet as a whole, in order to meet the CAFE (Corporate Average Fuel Economy) targets of a car manufacturer.

It is possible for oils designed for diesel engine service to also meet gasoline engine service. For these oils the designation is “C” category first followed by the “S” category. (see image below) “C” category oils have been formulated primarily for diesel engines and may not provide all of the performance requirements consistent with vehicle manufacturers’ recommendations for gasoline-fueled engines.

The lubricant formulas from AMSOIL INC., who produced the world’s first API-qualified synthetic motor oil in 1972, are unique and proprietary. How does the company ensure that its products meet or exceed the minimum specifications of the tests required for API licensing?

First, AMSOIL works closely with major additive companies to select the top-performing, and sometimes most expensive, passenger car and heavy-duty diesel motor oil additives. These additives have already passed all of the API licensing requirements in a petroleum- or synthetic-based formulation.

Then, it works with the additive company to maximize the amount of additive used and to boost the additive package in selected performance areas to achieve an optimum performing additive package for the intended application.

This is unlike many other oil companies that, because additives are expensive, use the minimum amount of the least expensive additives required to meet the minimum API requirements.

It then uses a combination of synthetic base stocks with known performance characteristics as replacements for the petroleum base stocks to optimize performance in areas of lubricity, volatility, viscosity index, oxidation and nitration resistance, pour point, flash point, deposit control, soot handling, emissions and many other areas as well.

It also uses a high-quality Viscosity Index Improver (VII) with outstanding viscosity and cleanliness properties. This replaces the inexpensive, low-quality VII used in standard API-licensed petroleum formulas. These high-quality VIIs are far more "shear resistant" than lower quality ones.

AMSOIL does laboratory bench tests and runs field tests to verify the superiority of the synthetic formula. It continues to monitor the performance of the oil through close scrutiny of tens of thousands of oil analysis tests per year across a wide variety of vehicles all around North America and the world.

AMSOIL has been collecting used synthetic oil samples from passenger cars since 1982. No other oil company has such a vast database of the performance of synthetic lubricants over extended drain intervals.

Our lubricants and formulations (both API-licensed and non-API-licensed) outperform lower quality API-licensed oils because they are engineered for top performance from the very beginning. They far exceed the API minimum requirements.

Even so, AMSOIL has its own limited warranty, protecting you even further.

In 1911 the Society of Automotive Engineers (SAE) created a numerical code graduated system - the SAE J300 Engine Oil Viscosity Classification - to classify motor oil according to their viscosity characteristics.

The SAE wanted a system that reflected the suitability of an oil for use as an engine lubricant and was easy for the consumer to understand. Before the SAE came up with the SAE J300 system, there was no simple way to tell how motor oil would behave in a hot engine.

Initially, the first version of the SAE J300 EOVC system defined five different numbered grades for motor oil (SAE 10, 20, 30, 40, and 50) based on flow rates (viscosities) measured at 100°C. By 1926 there were six grades of oil defined (SAE 10 through SAE 60).

Over the years, as shortcomings were identified, the SAE J300 system was amended numerous times. For instance, four SAE "W" (Winter) grades (SAE 10W, 15W, 20W, and 25W) were added in 1952, which were specified by viscosity measured at -18°C (0°F), as it became apparent that engines could not be started in very cold weather. Two more low-temperature grades (0W and 5W) would later be added.

In the early 1970's, minimum High-Temperature/High-Shear (HT/HS) specifications (measured at 150°C) were added when it became obvious that engines suffered from excessive wear or even seized when operating at high temperatures under high load (e.g. high speeds, towing).

By this time, the SAE J300 Viscosity Classification system comprised eleven distinct motor oil viscosity classifications, six low-temperature grades (SAE 0W, 5W, 10W, 15W, 20W, and 25W) and five high-temperature grades (SAE 20, 30, 40, 50, and 60); the lower the number, the lower the temperature at which the oil could be used for safe and effective protection. The higher numbers reflected better protection for high heat and high load situations.

In the 1980's, there were several outbreaks of catastrophic engine failures in both the U.S. and Europe due to unusually cold weather. Some engine oils thickened and gelled in these conditions. Engines would start but their pumping systems were incapable of pulling the cold oil out of the oil pans. The result was a rash of engine failures, warranty claims, and motor oil recalls. To address this problem the J300 cold weather specification was modified to require cold temperature cranking and cold temperature pumping tests.

On April 2, 2013, another revision to the SAE J300 Engine Oil Viscosity Classification (EOVC) was published adding a new high-temperature viscosity grade (SAE 16) to the previous SAE J300 system. The J300 revision was requested by a consortium of passenger car OEMs to provide a viscosity grade lower than SAE 20 in order to meet increasingly stringent fuel economy requirements.

The new grade is specified by OEMs for cars specifically designed to use new low-viscosity oils. It is not deemed to be suitable for use with older engines or newer vehicles not designed for such low-viscosity oils. Increasingly lower oil viscosity grades could be defined in the future.*

* Update: SAE 0W-8 motor oil was added to the SAE J300 standard in January 2015, although it did not see widespread commercial availability or vehicle adoption until around 2023. Primarily developed for hybrid engines to improve fuel economy, it is often associated with the JASO GLV-1 specification. SAE 0W-12 motor oil was introduced to the market, particularly for BMW, in 2022, for use in select 2023 model year engines.

Besides adding the SAE 16 grade, the new revision also revised the minimum viscosity limit of SAE 20. In the past, an SAE 20 oil grade’s viscosity range, measured at 100°C, was from 5.6 cSt to 9.3 cSt, which was a much broader range than that of SAE 30, 40, 50, or 60 grades. Additionally, the lower part of the old SAE 20 range was not being utilized. Therefore, the minimum kinematic viscosity was increased from 5.6 cSt to 6.9 cSt to bring the range of SAE 20 in line with that of the higher-viscosity grades.

The SAE grade numbers are determined by the specific kinematic viscosity range (measured at 100°C) that a particular oil falls into. For example, a motor oil that has a kinematic viscosity of 10.4 cSt at 100°C will be classified as an SAE 30 grade oil since it falls within its viscosity range of 9.3 cSt to 12.5 cSt.

For the high-temperature viscosity grades, both minimum and maximum kinematic viscosity limits are given. However, for the low-temperature (“W”) grades, only the minimum kinematic viscosity limit is given because these grade numbers are primarily determined by the Low-Temperature Cranking and Low-Temperature Pumping apparent viscosity* measured at a specified temperature and shear rate.

* The Apparent Viscosity (AV) of a fluid, usually reported in units of centipoise (cP), is dependent upon shear conditions. The apparent viscosity changes with the rate of shear (i.e., apparent viscosity decreases as shear rate increases). Therefore, the shear conditions need to be specified in order to calculate the apparent viscosity measurements at a given temperature. (For more on the apparent viscosity of a fluid, see: Newtonian vs. Non-Newtonian)

The classifications increase numerically; the lower the number, the lower the temperature at which the oil can be used for safe and effective protection. The higher numbers reflect better protection for high heat and high load situations.

Note that an SAE 20 and an SAE 20W are two completely separate classifications. The "W" or “winter” rating indicates that the grade is suitable for use in cold temperatures. Single grade oils have a limited range of protection and, therefore, a limited number of uses. With today's well-refined, high viscosity index oils, however, an SAE 20 oil usually will meet the viscosity requirements of SAE 20W and vice versa. Those that do are classified SAE 20W-20. This multi-grade or multi-viscosity ability increases oil's usefulness, because it meets the requirements of two or more classifications.

Here are some more examples of multi-viscosity oils: SAE 0W-20, 5W-30, 10W-40, 20W-50. The number with the "W" designates the oil's properties at low temperatures, while the second number characterizes its properties at high temperatures.

For instance, a multi-viscosity SAE 5W-30 meets the 5W criteria when cold and the 30 criteria once hot. SAE 5W-30 has been widely used (although, today, SAE 5W-20 and 0W-20 have become increasingly more popular in newer vehicles) because under all but extremely hot or cold conditions, it is light enough for easy engine cranking at low temperatures and heavy enough to protect at high temperatures.

(For more on viscosity grades, see: "Motor Oil Viscosity Grades")

So far, the products discussed have been fluids, but what about greases? In some lubricant applications, it is impossible to contain a fluid lubricant. For these applications, greases are used.

A simple description of grease is a semi-solid lubricant composed of base fluid, additives and a thickener. The thickener in grease is added in most cases to help keep the lubricant in place on applications where a fluid lubricant would run off and only provide lubrication for a very short time.

The National Lubricating Grease Institute (NLGI) created a set of ranges that have become the standard by which most greases are produced, marketed and sold. These ranges characterize the flow properties of greases.

Because greases are not a fluid, their resistance to flow is generally called consistency instead of viscosity. Greases are sold by consistency grade.

Grease consistency is measured using the cone penetration test. (see images below)