Introduction to motor oils

Introduction to motor oils

Motor oil is one of the decisive factor affecting the performance of vehicle engines and the lifespan of engines. Therefore, the choice of the suitable motor oil for your car engine is very important.

Functions of motor oils

  • Lubricating the moving parts of engines to minimize metal to metal contact and abrasion.
  • Serving as a cooling agent which reduces the heat generated from the combustion and friction in the engine.
  • Stopping gas leak.
  • Reducing sludge and deposit formation.
  • Preventing engine parts from corrosion and rusting.

Composition of motor oils

Motor oils for vehicles are blended from a base oil and additives at a certain ratio.

Categorization of base oils

Lubricant base stocks are categorized into five groups (Group I, II, III, IV, V) by the API.
  • Group I base stocks are composed of fractionally distilled petroleum which is further refined with solvent extraction processes.
  • Group II base stocks are composed of fractionally distilled petroleum that has been hydro cracked to further refine and purify it.
  • Group III base stocks are produced by further hydro cracking of either Group II base stocks or hydro isomerized slack wax (a Group I and II dewaxing process by-product).
  • Group IV base stock are poly alpha olefins (PAO).
  • Group V is a catch-all group for any base stock not described by Groups I to IV. Examples of group V base stocks include polyolesters (POE) and polyalkylene glycols (PAG).
Group I oils are the cheapest in terms of price.

Motor oil additives

Additives included in the motor oils for improvement of their properties or additional benefits are called “lubricant additives” in general. Some common additives include:
(1) Oxidation Inhibitors or Anti-Oxidants
(2) Rust Preventives or Anti-Rust Additives
(3) Anti-Wear Additives
(4) Viscosity Index Improvers
(5) Detergents
(6) Dispersants
(7) Alkaline Agents
(8) Corrosion Inhibitors or Anti-Corrosion Additives
(9) Pour Point Depressant
(10) Oiliness Additives
(11) Extreme Pressure Additives
(12) Anti-Foaming Additives

Motor oil categorization

  • Mineral oil: A mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum. The main components are alkanes and cycloalkanes. Compared with synthetic oils, mineral oils have less stable molecular structures, lower fluidity at low temperature and weaker anti-oxidation as well as lubricating ability at high temperature. However, mineral oils are suitable for general operating conditions.
  • Semi-synthetic oil: Semi-synthetic oils are blends of mineral oil with no more than 30% synthetic oil. Semi-synthetic oils have more stable chemical molecules compared to mineral oils. Possessing advantages of both mineral and fully synthetic oils, semi-synthetic oils have more practical uses.
  • Fully synthetic oil: Fully synthetic oils are blended from 100% chemically synthetic base oils (ester or catalytic hydro processing oils) and additives, which contributes to their good anti-oxidation ability, higher fluidity at low temperature, high viscosity index, low volatility, and high stability against high temperatures. Fully synthetic oils are capable of achieving their optimal performance even under extreme operation conditions.

Regulations on viscosity of automobile motor oils

SAE viscosity grades (e.g. SAE 10W40)
The viscosity grade of a lube oil is determined by the Society of Automotive Engineers (SAE). The “W” in the viscosity grade refers to the low-temperature viscosity (“Winter”), which implies the easiness at start-up, e.g. 0W, 5W, 10W, 15W, 20W…. On the other hand, the number behind “W” in the viscosity grade refers to the oil film strength at high temperature, e.g. 20, 30, 40, 50, 60. The larger the viscosity grade, the wider the range of its suitable conditions. For example, 5W50 has a stronger oil film at high temperature compared to 15W40 and has a wider application range.

ACEA is the abbreviation of Association des Constructeurs Européens d'Automobiles (European Automobile Manufacturers’ Association). The committee developed classifications of engine oils in gasoline, passenger cars diesel and heavy duty diesel engines. “A” designates classifications for gasoline engines, including A1, A2, A3, and A5. “B” designates classifications for car diesel engines (light-duty diesel), including B1, B2, B3, B4, and B5. “E” represents classifications for heavy duty diesel, including E2, E3, E4, and E5.


API-ASTM is a car engine oil standard developed by API (American Petroleum Institute) and ASTM (American Society for Testing and Materials). Below are the new standards and their corresponding applications:
Motor oils for gasoline engines Motor oils for diesel engines
  • SA had no specific engine testing standards

  • SB was for engines in 1930 - 1939

  • SC was recommended for engines in 1964-1967

  • SD was recommended for engines in 1968-1970

  • SE was recommended for engines in 1971-1979

  • SF was recommended for engines in 1980-1988

  • SG was recommended for engines in 1989-1993

  • SH was recommended for engines in 1994-1996

  • SJ started to be used since October 15, 1996 and is applicable to all existing car engines
    SL started to be used since July, 2011 and is applicable to both new and old engine models. Oils meeting SL provides cleanness at high temperature and lower oil consumption. Oils meeting the ILSAC specification possess fuel saving properties.
    SM started to be used since November 30, 2004 and is applicable to both new and old engine models. Oils meeting SM provides even higher cleanness at high temperature and even lower oil consumption. The most important is these oils have a low-phosphor content and meet the requirements of anti-oxidation, anti-wear, and reducing deposits. If the oils meet the ILSAC CF4 level at the same time, energy-saving and carbon reduction are achieved.
    SN started to be used since October 01, 2010 and meets the latest energy-saving and carbon-reducing standard as well as the ILSAC oil specifications at the same time.
  • CA was for light-duty engines in 1940-1950

  • CB was for medium duty engines in 1949-1960

  • CC was for engines brought to the market after 1961

  • CD started from 1955 and was applicable to high quality diesel for compression engines

  • CE came to being in October 1987. Oils meeting CE had better anti-oxidation, anti-wear, deposit reducing, and fuel saving abilities compared to prior oils.

  • CF2 started from 1994 and is applicable to heavy-duty two-stroke engines. CF2 substitutes the CD II oils.
    CF4 started form 1990 and is applicable to naturally aspirated and turbocharging engines. CF4 substitutes CD and CE oils.
    CG4 started form 1995 and is applicable to heavy-duty, high-speed, four stroke engines whose fuel contains no more than 0.5% sulfur.
    CH4 started from 1998 and is applicable to high-speed, four stroke engines where the fuel sulfur content conforms to the regulations in 1998.
    CI4 was introduced in 2002 and conform to the emission regulations in 2004. CI4 is suitable for a new generation of diesel engines. New technologies such as gasoline direct injection (GDI) and common rail are applied in order to increase the combustion efficiency and be distinguish itself form catalytic converters
    CJ4 was introduced in 2007 with the view to meet the Euro 4 emission standards and the US environmental protection in year 2007. Specially suitable for engines with EGR and DPF whose diesel contains a low sulfur content (<15ppm).
● implies the engine testing is abolished. It is not assessed by ASTM any more except for special conditions.

Choosing the right viscosity

Nowadays the mainstream of motor oil in Taiwan are multi-grade oils. A right choice of the motor oil must be based on the usual operating conditions and the car condition.
Low temperature(W) 5W is suitable for use in Taiwan, recommended for new cars.

10W is suitable for use in Taiwan, recommended for regular cars or new ones.

15W is suitable for use in Taiwan, recommended for regular cars or older ones.

20W is suitable for use in Taiwan, recommended for older cars or the ones whose engine consumes motor oil.
High temperature 30 is suitable for special engines for which an original standard motor oil viscosity is required.

40 is suitable for use in Taiwan, recommended for regular cars or new ones.

50 is suitable for use in Taiwan, recommended for regular cars or those under extreme operating conditions.

60 is suitable for heavily modified cars or special cars with high power.
◎ To extend the life of a new car engine, fully synthetic multi-grade oils are highly recommended. Relatively inexpensive semi-synthetic oils can be a good choice for older models.

Oil Pressure

1. How to tell the quality of a motor oil: In theory the viscosity of motor oils increases with pressure rise. In contrary an increased oil temperature due to extreme operations could reduce the oil pressure. Therefore, maintaining a stable oil pressure is an essential way to make sure that the motor oil has proper viscosity and oil film strength for lubricating the engine parts.

2. Most people monitor the motor oil pressure based on the oil pressure gauge in car. Upon changing new oil, the oil pressure is recorded. The fact that the oil pressure decreases significantly over time or after extreme operations implies a reduction of oil viscosity, which in turn means that the oil film strength may not be enough to protect the engine. However, it’s noteworthy that, the oil pressure is mainly generated by the motor oil pump, so the oil pressure of a single motor oil can vary in different cars. Judging the quality of a motor oil merely by the pressure value upon changing oil might also be improper, since the various formula and additives of motor oils could result in high fluidity and excellent oil film strength at the same time; on the other hand, some motor oil having high oil pressure might also have poor fluidity and lubricating ability, leading to higher resistance to flow and slowing down engine acceleration.

3. Procedures for engine oil pressure testing:

Step 1
To ensure fairness and impartiality, motor oils with the same viscosity index (e.g. SAE 5W50) should be chosen if oils of different brands are under examination. A new oil filter must be used for each product.

Step 2
Record the oil pressure at idle and the maximum oil pressure as soon as the oil temperature of 80°C is reached after changing oil and starting the engine. Then record the time to accelerate the engine at full gas from idle speed to the maximum speed where oil is no longer supplied. The results indicates the lubricating ability of a motor oil. (Shorter time is required for oils with better lubricating ability.)

Step 3
Noise test. The noise value at various speeds are acquired from a sound meter installed at 30 cm from the central line of an engine. Noise is measured at 4 different speeds: idle speed, 3000 rpm, 5000 rpm, and 6500 rpm. The lower the noise value, the better the lubricating ability of the motor oil.

Step 4
Examine the maximum power using various motor oils on a dynamometer. Afterwards do a severe road simulation on the dynamometer with a continuous operation in 3rd gear at 6500 rpm and 150 km/h for 3 minutes and observe the oil pressure and temperature change at each point of time.

◎ Conclusion:The quality of a motor oil cannot be determined merely by the oil pressure. However, a good motor oil should be able to maintain a stable oil pressure at any temperature or under extreme operations and should possess an excellent lubricating ability as well as a strong oil film.