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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.

Cold Cranking Viscosity (CCV) affects the 'startability' of engines in cold temperatures. Low cold cranking viscosities make for easier cold temperature cranking and starting, resulting in less engine wear and less drain on the battery.

The Cold-Cranking Test determines if an engine can be cranked over fast enough to start under extreme cold ambient conditions. Cold Cranking viscosity is determined by using a Cold-Cranking Simulator (CCS). (see image below)

A Cold-Cranking Simulator simulates an oil's cranking resistance when cold, such as the viscosity of an oil in crankshaft bearings during start up on a cold winter morning, thus indicating the lowest temperature at which an engine is likely to start. The resulting "apparent" viscosity is measured at a specified temperature and shear rate and is usually reported in units of centipoise (cP).

Cold Pumping Viscosity measures the resistance of an oil to pumping through the engine after a cold start. If an oil's viscosity becomes too high (if the oil is too thick), pumping will be hindered with possible cavitation issues. Viscosity here becomes an important factor in determining whether the engine runs with sufficient lubrication after starting in severe cold conditions.

From an engine durability perspective, the most important low temperature oil performance issue is pumping, to ensure that the oil can circulate after the engine fires. Oil that is too thick in these conditions can cause oil starvation which could result in significant wear in critical engine parts.

The Cold Pumpability test is always conducted at 5°C colder than the Cold Cranking test to ensure the pump can deliver the oil to the bearings. Cold Pumping viscosity is determined by using a Mini-Rotary Viscometer (MRV). (see image below)

In this test method, oil is cooled slowly through a temperature range in which wax crystallization is known to occur, followed by rapid cooling to the final test temperature. The resulting "apparent" viscosity is measured and is reported in units of centipoise (cP).

Correlations have been found between lack of pumpability in real field applications and failures in this test. These failures in the field are thought to be the result of the oil forming a gel structure that results in excessive yield stress or viscosity of the engine oil, or both.

High-temperature/High-shear (HT/HS) Viscosity (measured at 150°C) is a measure of an oil's ability to retain its viscosity and resist shearing when an engine is operating under load at high-temperatures (in severe service conditions). An oil that is too thin under these conditions may not provide the needed lubricant protection, which could result in significant wear in these critical engine parts.

An oil’s film thickness can be severely affected when an engine is exposed to high temperatures and the high shearing forces that are created when an engine is operating under load. These high temperatures and shearing forces can cause an oil to thin out and lose its load carrying ability.

The oil’s "apparent" viscosity varies inversely with the rate of shear to which it has been subjected to, that is, as the rate of shear increases, the viscosity of the oil decreases. This is referred to as shear-thinning.

The HT/HS Viscosity test is designed to determine the apparent viscosity of an oil under conditions of high shear at high temperatures. The resulting viscosity, measured at 150°C, is usually reported in units of centipoise (cP) and is determined by using a High-Temperature High-Shear Capillary Viscometer. (see image below)

The Oil Viscosity Grade Numbers (e.g. the 10W and the 30 in 10W-30 multigrade oil – sometimes referred to as the “weight” of the oil) are a rating representing the viscosity range and the viscosity limits of the oil. They are NOT the actual viscosity of the oil.

As mentioned above, the viscosity of oil is temperature dependent. In other words, a particular grade of oil will have a different viscosity/thickness at different temperatures; and since viscosity varies with temperature, the temperature must be clearly specified in order to interpret the viscosity reading.

For example, let’s look at an SAE 30 grade oil and how its viscosity varies with temperature. At normal operating temperatures (100°C) it has a kinematic viscosity* of around 10 cSt (centistokes) – which is optimal for most engines. But at 24°C (room/ambient temperature), it has a viscosity of around 250 cSt (which is way too thick).

* Again, kinematic viscosity, which is determined by using a Capillary Tube Viscometer. (see image below), is the amount of time, in centistokes (cSt or mm2/s), that it takes for a specified volume of lubricant to flow, under the force of gravity, through a fixed diameter orifice at a given temperature.

Now let's consider a 10W grade oil and how the viscosity of this grade of oil varies with temperature. At normal operating temperatures (100°C) it has a kinematic viscosity of around 5 cSt (way too thin for a modern engine). But at 24°C, it has a viscosity of around 30 cSt (a lot better than 250 cSt).

In other words, what this means is that a 30 grade oil has pretty close to an ideal viscosity (around 10 cSt) at operating temperatures but is way too thick when cold (around 250 cSt) whereas a 10W grade oil has a more acceptable/desirable viscosity (around 30 cSt) when cold but is way too thin (around 5 cSt) at operating temperatures.

In order to address this problem, oil companies, in the 1940s, came up with Multi-grade oils (sometimes described as all-season oils). The benefits of multi-grade (or multigrade) oils when starting the engine at lower temperatures was immediately apparent to many car owners and the popularity of these engine oils grew rapidly.

Multi-grade oils made it possible for an oil to meet both the low-temperature and the high-temperature grade specs. Let's use a conventional (mineral based) SAE 10W-30 multigrade oil as our example.

Because a 10W grade oil becomes way too thin at normal operating temperatures (having a viscosity of around 5 cSt when it should be around 10 cSt) what the oil companies did to solve this problem is add Viscosity Index Improvers to it to prevent it from thinning as much as it normally would as it gets hotter. In other words, VII slow down the rate at which oil thins out as the temperature rises.

Now instead of having a viscosity of around 5 cSt when hot, it now has a viscosity of about 10 cSt (the same as an SAE 30 oil). It still remains a 10W grade oil but now behaves like a 30 grade oil when hot.

Viscosity Index Improvers (VII) are large oil-soluble polymers made up of long-chain flexible molecules of high molecular weight. (see image below) They are sometimes referred to as Viscosity Modifiers (VM). An example of a VII is an Olefin Co-polymer (OCP) which is a co-polymer of ethylene and propylene.

VII polymers expand and contract (coil up and uncoil) as temperatures vary. High temperatures (e.g., when the engine oil is heated up to operating temperature) cause VII to expand (uncoil and stretch out) and reduce oil thinning (i.e., they interact with the layers of oil that are moving relative to two metal surfaces creating more resistance to flow, which is known as 'viscosity'.)

Low temperatures, conversely, cause the VII polymers to contract (coil up tightly) and have little impact on oil viscosity (see illustration below).

Imagine a new deck of cards on a table. Your hand pushes the top card horizontally. The top card slides with your hand, the bottom card stays where it was on the table, and the cards in between slide over each other at a gradient of distance proportional to the distance from either the table or your hand. Now imagine the cards are made of cardboard versus plastic, and a bit “rough” on their surfaces. The roughness of one card interacts with the adjacent card to impede movement more so than the cards of the smooth plastic deck of cards. The uncoiled polymers of viscosity index improvers (VIIs) in oil act like the roughness of the cardboard cards to provide resistance to flow.

A finished multi-grade oil is formulated with Viscosity Modifiers (VMs) in a low viscosity base oil. The base oil viscosity meets the targeted low temperature viscosity of the finished lubricant. The base oil viscosity, although relatively high at cold temperatures, decreases as the oil is brought up to operating temperature. As the temperature rises, the VMs uncoil, "stretch out" and interact with neighboring layers of the oil to slow their movement causing more 'resistance to flow', or an increase in viscosity. The viscosity increase from the VMs with increased temperature, however, is not enough to offset the decrease in viscosity of the base oil. Thus, even though an SAE 5W-30 behaves like an SAE 5W at low temperatures and like an SAE 30 at operating temperatures, the oil’s viscosity (thickness) is still less at operating temperature versus at low temperatures.

Viscosity Index Improvers come in different shapes, sizes and quality levels (i.e. some polymer chemistries are better thickeners than others). Generally speaking, larger molecules are better thickeners than smaller ones; however, they are also more easily broken, which impacts the shear stability of the oil (see following section).

To make sure the VII is used in the most cost-effective way, polymer thickening efficiency is also important. Thickening Efficiency (TE) describes the boost in Kinematic Viscosity at 100°C of an oil following the addition of a specific amount of VII polymer. A VII polymer having a high TE indicates that it is a potent thickener. TE is primarily a function of polymer chemistry and molecular weight.

The downside to mineral-based multigrade oils is that they require a lot more VII polymer additives in order to meet the proper viscosity requirements. Problem is, the VII polymer additive wears out (shears) over time, effectively reducing the oil’s viscosity, until the motor oil becomes too thin to provide adequate protection (which is one of the reasons a conventional motor oil needs to be changed more often than a synthetic motor oil).

Synthetic motor oils on the other hand, such as AMSOIL premium synthetic motor oils, have a much higher native viscosity index than conventional motor oils and therefore need much lower levels of VII polymer additives (in some cases, none at all) to increase their viscosity to the desired levels. As a result, they are able to maintain their proper viscosity for a much longer period of time and therefore don’t need to be changed as often as conventional motor oils.

High Temperature/High Shear (HT/HS) and Permanent Shear Stability Index (PSSI) are two critical characteristics for improving efficiency and preventing wear. Understanding these characteristics is critical for OEMs to meet higher and higher fuel economy requirements and for you to make sure you choose the right oil to protect your engine from premature wear.

The VIIs react to temperature but also are affected by shear and exhibit both temporary and permanent shear characteristics. Oil experiences very high stresses in certain areas of the engine such as in the oil pump, cam shaft area, piston rings, and any other area where two mating surfaces squeeze the oil film out momentarily. An oil’s viscosity can be severely affected when an engine is exposed to high temperatures and the high shearing forces (HT/HS) that are created when an engine is operating under load. These high temperatures and shearing forces can cause an oil to thin out and lose its load carrying ability, resulting in engine wear.

The oil’s viscosity varies inversely with the rate of shear to which it has been subjected to, that is, as the rate of shear increases, the viscosity of the oil decreases.

As previously mentioned, multigrade engine oils contain Viscosity Index Improvers (VII) which are made-up of very large “viscosity-altering” oil-soluble molecules also known as Viscosity Modifiers (VM). As they are heated, these large flexible polymer molecules uncoil and stretch out. Their many “branches” entangle with those of other polymer molecules and trap and control many tiny oil molecules. (see illustration below)

As oil is squeezed between two mating surfaces, such as between a bearing and a journal, VII polymer molecules, or coils, have a tendency to align with each other and get "squashed" or "compressed". This causes the polymer coil to deform (elongate/stretch) and become aligned to the direction of flow.

When this happens, viscosity temporarily drops resulting in oil film thickness reduction. After the oil progresses through the bearing, the polymer coils “spring back” to their original shape and viscosity returns to normal. This phenomenon is referred to as temporary shear-thinning.

In certain high shear conditions, the long flexible polymer chains can be ruptured or pulled/ripped apart (molecular scission) into smaller chains. When the shearing force is removed, the broken polymer chains cannot re-form into the single large chain because the coil has been physically and chemically changed.

Consequently, this causes the oil to permanently lose viscosity leading to a reduction of oil film thickness which can lead to oil film failure and an increase in engine wear. For example, an SAE 5W-30 oil can ‘shear back’ to an SAE 5W-20 oil or worse, resulting in damaging deposits, increased wear, and reduced engine life. This phenomenon is referred to as permanent shear-thinning.

The ability of an oil to resist these shearing forces is called shear stability.

Shear Stability is a measure of the resistance of an oil to changes in viscosity caused by the oil being subjected to mechanical stress or shear. The outcome of this mechanical shear is a reduction in viscosity resulting in reduced oil film thickness allowing the possibility of metal to metal contact.

Needless to say, an oil’s shear stability – its ability to retain its viscosity in High-Temperature/High-Shear (HT/HS) conditions – is essential in maintaining an adequate oil film thickness between moving surfaces, effectively preventing metal to metal contact. Viscosity retention of multi-grade oils is an important performance characteristic since adequate oil film thickness is required for engine wear protection.

VII polymers having higher molecular weight have a greater propensity for polymer coil breakage. In other words, while large molecules are better thickeners then smaller ones, they are also more easily broken, which impacts the shear stability of the oil.

Moreover, different VII polymers have different shear stability characteristics, depending on their molecular weight and chemical nature, and come with different Shear Stability Indexes that reflects their quality and capability to resist permanent viscosity loss.

A polymer’s Shear Stability Index (SSI) is defined as its resistance to mechanical degradation (polymer coil breakage) under shearing stress; the lower the polymer’s SSI the better the oil’s ability to resist shear and suffer permanent viscosity loss. PSSI is a measure of how much a VII permanently shears under specific conditions. A VII with a PSSI of 10 loses 10% of its viscosity contribution to the oil's viscosity, and a VII with a PSSI of 50 loses 50%. Therefore, the lower the PSSI, the more stable the VII is.

There exist several different ASTM methods for measuring Permanent Shear Stability. So, when comparing PSSIs of lubricants, it is critical to know the test method used.

Some procedures pass the oil through a fuel injector, and different ASTM test methods require different pressures and number of passes through the injector. Another two ASTM test methods utilize a sonic oscillator to cavitate the oil creating the shearing mechanism. Although the shearing mechanism of the sonic and the injector methods are different, the results do correlate fairly well with each other. A more severe shear test is the KRL test, which utilizes tapered roller bearings under load for a specified duration and rpm. The KRL test is typically used for gear applications, whereas the injector and sonic tests are commonly used for engine and hydraulic lubricants.

The Kurt Orbahn Diesel Injector Test (see image below) is a frequently used test to quantify Permanent Shear Stability. It measures the permanent reduction in an oil’s viscosity after 30 cycles, or in some cases 90 cycles, through the test apparatus. This mechanical process mimics high-shear, real-world engine conditions to evaluate how much the viscosity modifier polymer breaks down (shears), causing permanent viscosity loss. The lubricant is forced through a specific diesel injector nozzle at a pressure of 13 to 18 MPa. This is done for 30 cycles (ASTM D6278) or sometimes 90 cycles for more severe, long-drain interval testing.

The Kurt Orbahn test is generally considered less severe than the Sonic Shear or KRL (tapered roller bearing) methods, but it correlates well with field data. The motor oil’s viscosity falls during the test due to polymer coil breakage (permanent shear). In other words, only that part of the oil’s viscosity which is contributed by the VII polymer is susceptible to breakage. Neither the base oil nor the additive performance package suffers permanent viscosity loss.

For example, an oil is formulated with a base oil viscosity of 5 cSt (at 100°C) and a Viscosity Index Improver (VII) is used to increase its viscosity to 15 cSt. The VII’s viscosity contribution is therefore 10 cSt. During the shear test, the oil’s viscosity falls to 12 cSt. It has permanently lost 3 cSt of viscosity. The VII polymer’s ‘Shear Stability Index’ (SSI) is therefore 3 cSt (loss) divided by 10 cSt (VII contribution), or 30% SSI. (see illustration below)

Shear stabilities of VIIs are inversely proportional to their molecular size. Generally, larger, or longer, polymers are more susceptible to shearing compared to shorter polymers. The longer polymers are more efficient in providing the required VI, in that it takes a lower dosage of the long polymer VII than the shorter more durable polymers, and, thus, the cost of the finished lubricant can be lower. This is partially why the early versions of multigrade oils were known for not staying "in grade".

The early multigrade finished lubricants utilized the new-at-the-time polymers to enhance the VI of engine oils, but it quickly became evident more durable VIIs were needed. Shorter polymers improve the durability issue but require a higher dosage, and, thus, higher cost, to acquire the same level viscosity modification, but they do stay "in grade". The appropriate compromise of efficiency versus durability, and cost, must be considered for the finished lubricant life in a given application. Extended life lubricants, such as AMSOIL's premium top-tier lubricants, require more durable polymers to be able to provide protection over the life of the lubricant.

Note also that synthetic lubricants have much higher native viscosity indices than conventional oils and therefore do not rely as much on “polymeric thickeners” to attain their desired viscosities. As a result, they experience very little viscosity loss due to shearing making them very shear stable and able to better protect and remain "in-grade" for a much longer period of time. This is one of the reasons why synthetic oil can remain 'in service' for an extended change interval.

(see also: How Often Should I Change the Oil?)

Our AMSOIL top-tier synthetic motor oils, for instance, are formulated using the industry's most advanced synthetic base stocks, additive packages, and Viscosity Modifiers allowing them to significantly outperform and outlast conventional oils and most other synthetic oils. (They’re also sugar free and low in fat 😊 )