There are two main classification criteria for motor oils:

Based on Viscosity (SAE)

Based on Performance (API, ACEA, manufacturer’s specifications)

Kinematic viscosity measured at 100°C defines SAE degrees from 20 to 60 for rising levels of viscosity. Dynamic viscosity at low temperatures defines the SAE “W” degrees, from the initial “winter”, from 0W to 25W on the basis of viscosity levels measured at temperatures from -35° to -5°C. The temperature represents the lowest possible temperature at which the engine can be started when lubricated with an oil of the corresponding SAE degree (e.g. a 15W oil makes it possible to start the engine at up to -20°C). The minimum pumping temperature is the minimum temperature at which oil, in addition to allowing start up, can flow freely and lubricate the critical parts of the engine.

The most widely used system for engine oil viscosity classification is that established by the Society of Automotive Engineers (SAE) in the USA. In this system two series of viscosity grades are defined – those containing the letter W and those without the letter W. Grades with the letter W are intended for use at lower temperatures and are based on a maximum low temperature viscosity and a maximum borderline pumping temperature, as well as a minimum viscosity at 100°C. The low temperature viscosity is measured by means of a multi-temperature version of ASTM D2602 ‘Method of Test for apparent Viscosity of Motor Oils at Low temperature using the Cold Cranking Simulator’.

Viscosities measured by this method have been found to correlate with engine speeds developed during low temperature cranking. Borderline pumping temperature is measured according to ASTM D3829 ‘Standard Method for Predicting the Borderline Pumping Temperature of Engine Oil’. This provides a measure of an oils’ ability to flow to the engine oil pump inlet and provide adequate engine oil pressure during the initial stages of operation. Oils without the letter W, intended for use at higher temperatures, are based on the viscosity at 100°C only. These are measured by ASTM D445 ‘Method of Test for Kinematic Viscosity of temperature and Opaque Liquids’. ‘multi-grade’ oil is one whose low temperature viscosity and borderline temperature satisfy the requirements of one of the W grades and whose viscosity at 100°C is within the stipulated range of one-W-grades.

API stands for American Petroleum Institute. In 1970 along with the SAE and ASTM (American Society for Testing and Materials), they established the API Service Classification System to define the performance level of a given oil, unrelated in the main, to oil viscosity. The API requirements “S” for Spark Ignition (petrol) and “C” for Compression Ignition (diesel) can be briefly described as follows. For automotive gasoline engines, the latest engine oil service category includes the performance properties of each earlier category. If an automotive owner’s manual calls for API SJ or SL oil, API SM oil will provide full protection. For diesel engines, the latest category usually – but not always – includes the performance properties of an earlier category. Through the years, lubricant users have been treated to a number of ways to designate viscosity grades of the lubricants used in manufacturing. There are SAE (Society of Automotive Engineers) grades for gear oils and crankcases (engines), AGMA (American Gear Manufacturers Association) grades for gear oils, SUS (Say bolt Universal Seconds), cSt (kinematic viscosity in centistokes), and absolute viscosity. To add to the confusion, two measures of temperature (Fahrenheit and Celsius) can be applied to most of these, not to mention that viscosity might be presented at either 40°C (104°F) or 100°C (212°F). While all of these have served useful purposes to one degree or another, most lubrication practitioners settle on and use one method as a basis for selecting products. To the new entrant into the lubrication field, the number of options can be confusing, particularly if the primary lubricant supplier does not associate one of the prominent viscosity systems to the product label. To complicate matters, machinery designers must define the lubricant viscosity in such a way that the equipment user understands clearly what is needed without having to consult outside advice. These points to the need for a universally accepted viscosity designation – one that can be used by lubrication practitioners, lubricant suppliers and machinery design engineers simultaneously with minimal confusion.

ACEA stands for Association des Constructers Europeans de l’Automobile. This classification system is the European equivalent of the API classification system, but is stricter and has more severe requirements. Hence oil that meets both API and ACEA specifications uses a better additive package than one that is designed to meet only API specifications. Unlike the API, ACEA has three main groups – “A/B” for gasoline and light duty (passenger car, 4WD etc) diesel engines, “C” for light duty three way catalyst (TWC) and diesel particulate filter (DPF) compatible oils and “E” for heavy duty diesel engines. These can be defined as follows. The ACEA 2008 European Oil Sequences for Service-fill Oils comprise 3 sets (classes) of sequences: one for Gasoline and Light-Duty Diesel engines; one specifically for Gasoline and Light-Duty Diesel engines with after treatment devices and one for Heavy-Duty Diesel engines. Within each of these sets there are categories which reflect different performance requirements – four (A1/B1, A3/B3, A3/B4 & A5/B5) for gasoline and lightduty diesel engines; four (C1, C2, C3, C4) specifically for engines with after treatment devices, and four (E4, E6, E7, E9) for heavy-duty diesel engines. Specific applications of each sequence are the responsibility of individual engine manufacturers for their own vehicles / engines. The sequences define the minimum quality level of a product for self-certification to EELQMS and presentation to ACEA members. Performance parameters other than those covered by the tests shown or more stringent limits may be indicated by individual ACEA member companies. Where claims are made that Oil performance meets the requirements of the ACEA sequences (e.g. product literature, packaging, labels) they must specify the ACEA Class and Category (see Nomenclature & ACEA Process for definitions).

ILSAC (International Lubricants Standardization and Approval Committee) includes the major automobile manufacturers that manufacture vehicles in the USA. This includes the Japanese manufacturers. Effectively, ILSAC specifications are the fuel economy version of the API specifications. GF-1 is obsolete GF-2 is equivalent to API SJ GF-3 is equivalent to API SL GF-4 is equivalent to API SM GF-5 is equivalent to API SN ILSAC grades only apply to viscosities XW-20 and XW-30. GF-4 has introduced a phosphorus limit of 0.08% maximum and a sulphur limit of 0.2% maximum, GF-5 is similar, but it introduced new requirements relating to phosphorus volatility and compatibility with ethanol fuels. ILSAC, API and ACEA specifications require a large range of engine tests and laboratory tests on the oil. Parameters such as high and low temperature wear, oxidation, soot control, oil thickening, deposit control, volatility, stay in grade performance, fuel economy, chemical composition and many others are tested against limits and rated. In the case of the API, the oil specifications become more severe as the letters climb the alphabet, eg SL is more severe than SJ. This is not necessarily the case with ACEA as their specifications are more application specific.

GM-LL-A-025: Special GM approval for long-life engine oil for petrol engines. Viscosity is SAE 0W-30. Product meets ACEA A3/B3. Drain interval can be as long as 30,000 kms. Recommended for vehicles built before MY2011.

GM-LL-B-025: Special GM approval for long-life engine oil for diesel engines. Viscosity is SAE 5W-40. Product meets ACEA A3/B3/B4. Drain interval can be as long as 50 000 kms. Recommended for vehicles built before MY2011. GM DEXOSTM Motor Oil Specifications: GM DexosTM approved oils are  recommended to be used in all GM vehicles except those built with Duramax diesel engines that require an API CJ-4 quality oil.

GM DEXOS1TM: Designed with petrol engines from MY 2011 onwards, GM Dexos1TM replaces the GM-LL-A-025, GM6094M & GM4718M specifications. This specification is usually recommended for GM vehicles built for the North American and Asian markets. Compared to ILSAC GF-5, it has stricter requirements regarding piston deposit formation, aeration, oxidation stability, wear, low-temperature pumpability and volatility. SAE 0W-20, 5W-20 & 5W-30.

GM DEXOS2TM: The GM Dexos2TM specification is meant to be the replacement for both GM-LL-A-025 (petrol) & GM-LL-B-025 (diesel) specifications for the European market. Oils meeting GM Dexos2TM are required for vehicles manufactured from MY2011 onward but they are also backwards compatible with older models. This specification is built on the ACEA C3 standard but also contains elements from the ILSAC GF-4 deposit formation test and low-temperature sludge build-up test. SAE 0W-30, 5W-30, 0W-40 & 5W-40.

dexos1™ is GM‟s new global engine oil specification for gasoline engines which will replace GM 4718M and GM 6049M. Although it is being specified for all GM vehicles starting with the 2011 model year, it is also an excellent choice for previous model year vehicles. Oils meeting the dexos1™ specification exhibit enhanced performance compared to many oils on the market today. In fact, the dexos1™ specification is made up of a combination of some of the most demanding tests found in industry specifications such as those from API (American Petroleum Institute) and ACEA (European Automobile Manufacturers Association). The dexos™ specification sets a high standard, requiring both improvements in fuel economy and fuel economy retention over the life of the oil while at the same time requiring improvements in oil robustness. It is also designed to perform where GM‟s engine technology requires enhanced performance to operate at its best. In making a shift from how engine oil specifications are traditionally set, GM is providing for the future with a means to design and specify oils for its engines and engine technology as well as take full advantage of features such as the Engine Oil Life System (EOLS).

Compared to GF-5, the dexos™ weighted piston deposit (WPD) limit has been increased from 4.0 to 4.5. This translates to „cleaner pistons‟ and better engine performance. How? Piston deposits can form behind and around piston rings causing the rings to stick and be “sluggish”. Full compression does not occur resulting in increased emissions, decreased fuel economy and decreased engine performance overall. The dexos™ specification includes an aeration test on new and used oil. No such test is included in the next generation, GF-5 industry specification. Sufficient aeration control is a key criteria when the motor oil also functions as a hydraulic fluid, which is the case in GM engines designed with Variable Valve Timing. Variable Valve Timing (also referred to as cam phasing) is an increasingly common design in GM engines to improve fuel efficiency. The dexos™ specification includes a test for low-temperature pumpability. This sort of test is not currently included in the next generation industry specification. Although generally not an issue in warmer climates, if an oil becomes too viscous in colder temperatures, adequate lubrication may not occur resulting in increased wear and in severe cases, engine failure.