 Grades of motor oil
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Single-grade motor oil
The Society of Automotive Engineers, usually abbreviated as SAE, has established a numerical code system for grading motor oils according to their kinematic viscosity. For single-grade oils, the kinematic viscosity is measured at a reference temperature of 100 °C (212 °F) in units of mm²/s or the equivalent older non-SI units, centistokes (abbreviated cSt). Based on the range of viscosity the oil falls in at that temperature, the oil is graded as an SAE number 0, 5, 10, 20, 30, 40, 50, 60 or 70. The higher the viscosity, the higher the SAE grade number is. These numbers are often referred to as the weight of a motor oil. The reference temperature is meant to approximate the operating temperature to which motor oil is exposed in an engine.
The viscosity of single-grade oil derived from petroleum unimproved with additives changes considerably with temperature. As the temperature increases, the viscosity of the oil decreases logarithmically in a relatively predictable manner. On single-grade oils, viscosity testing can be done at cold winter (W) temperature (as well as checking minimum viscosity at 100 °C or 212 °F) to grade an oil as SAE number 0W, 5W, 10W, 15W, 20W, or 25W. A single-grade oil graded at the hot temperature is expected to test into the corresponding grade at the winter temperature; i.e. a 10 grade oil should correspond to a 10W oil. For some applications, such as when the temperature ranges in use are not very wide, single-grade motor oil is satisfactory; for example, lawn mower engines.
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Multi-grade motor oil
The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather. The difference in viscosities for any single-grade oil is too large between the extremes of temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers are added to the oil. These additives make the oil a multi-grade motor oil. The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened. This slope representing the change with temperature depends on the nature and amount of the additives to the base oil.
The API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. The first number associated with the W is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as cold as a single-grade 10 weight oil can be pumped. "5W" can be pumped at a lower temperature than "10W". "0W" can be pumped at a lower temperature than "5W", and thins less at temperatures above 99°C (210°F). The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle.
Some new vehicles are marked to use 0W25 oil. Some ultra fuel efficient and hybrid vehicles are marked to use 0W20 oil.
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Common multi-grade oils
Some of the common multi-grade oils are:
0W-20
0W-30
0W-40
5W-20
5W-25
5W-30
5W-40
10W-30
10W-40
15W-40
20W-40
20W-50
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Turbine motor oil
Turbine motor oils are designed somewhat differently than reciprocating engine oils traditionally used in automobiles. Deposit control and corrosion are not significant issues when formulating a turbine oil, and the shear stresses that turbine oils are exposed to are minimal in light of the fact that turbines are naturally balanced rotating machines unlike reciprocating engines. Turbine oils tend to have the ISO VG range 32, 46, and 68 (cSt at 40°C), and make extensive use of polyolester, polyalphaolefin, and Group II as base stock due to the high temperatures they must endure.
In most aviation gas turbine applications, peak lubricant temperatures are not reached during engine operation, but after shutdown, when heat has been able to migrate from the combustor cans and the compressors into the regions of the engine with lubricated bearings and gearboxes. The gas flow associated with running the turbine provides significant convective cooling that disappears when the engine is shut down, leaving residual heat that causes temperatures within the turbine to rise dramatically, an often-misunderstood phenomenon.
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American Petroleum Institute
Motor oil used for vehicle engines is commonly called engine oil in American Petroleum Institute (API) documentation. Engine oil is used for the lubrication, cooling, and cleaning of internal combustion engines. Motor oil may be composed of a lubricant base stock only in the case of non-detergent oil, or a lubricant base stock plus additives to improve the oil's detergency, extreme pressure performance, and ability to inhibit corrosion of engine parts. Lubricant base stocks are categorized into five groups by the API. Group I base stocks are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve certain properties such as oxidation resistance and to remove wax. Group II base stocks are composed of fractionally distilled petroleum that has been hydrocracked to further refine and purify it. Group III base stocks have similar characteristics to Group II base stocks, except that Group III base stocks have higher viscosity indexes. Group III base stocks are produced by further hydrocracking of Group II base stocks, or of hydroisomerized slack wax, (a byproduct of the dewaxing process). Group IV base stock are polyalphaolefins (PAOs). Group V is a catch all group for any other synthetic and mineral base stocks. Examples of group V base stocks include polyol esters, polyalkylene glycols (PAG oils), and perfluoropolyalkylethers (PFPAEs). Groups I, II, and III are sometimes referred to as mineral oils and groups IV and V as synthetic oils. However, some manufacturers (e.g., Castrol) have recently labeled their group III based oils as synthetic in the US, due to disputes over the definition of synthetic, and its application to motor oil.
Motor oils are further categorized by their API service class[1]. The API service classes have two general classifications: S for Service (typical passenger cars and light trucks using gasoline engines) and C for commercial applications (typical diesel equipment). The latest API service standard designation is SM for gasoline engines. The SM standard refers to a group of laboratory and engine tests, including the latest series for control of high-temperature deposits. Current API service categories include SM, SL and SJ for gasoline engines. All previous service designations are obsolete. There are six diesel engine service designations which are current: CI-4, CH-4, CG-4, CF-4, CF-2, and CF. All others are obsolete. It is possible for an oil to conform to both the gasoline and diesel standards. Engine oil which has been tested and meets the API standards has the API starburst symbol with the service designation on containers sold to oil users.
The International Lubricant Standardization and Approval Committee (ILSAC) also has standards for motor oil. Their latest standard, GF-4[2] was approved in 2004. A key test is the Sequence IIIG [3] , which involves running a 3.8L, GM 3.8L V-6 at 125 horsepower, 3600 rpm, and 150°C oil temperature for 100 hours. These are much more severe conditions than any passenger car would see. Street autos typically average a few dozen horsepower and 80°C. The IIIG test is about 50% more difficult [4] than the previous IIIF test, used in GF-3 and API SL oils.
The ACEA A3/A5, and MB 229.5 tests used in Europe are even tougher, it is debatable whether this matters for normal drain intervals (5000-7000 miles).
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Maintenance
In engines, there is inevitably some exposure of the oil to products of internal combustion, and microscopic coke particles from black soot accumulate in the oil during operation. Also the rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against the part surfaces causing erosion and wear. The oil filter removes many of the particles, but eventually the oil filter gets filled up. The motor oil and especially the additives also undergo thermal and mechanical degradation. For these reasons, the oil and the oil filter need to be periodically replaced.
The vehicle manufacturer specifies which grade of oil should be used for the vehicles it produces. The manufacturer also specifies how often the oil changes should be made. For example, most people in the United States believe that a common oil change frequency should be every 3000 miles or every 3 months, whichever comes sooner. This 3000 mile oil change interval has been relentlessly promoted by oil changing companies for decades. It had a scientific basis when engines used non-multi-weight, non-detergent oil. It no longer has any scientific basis, but it is still being promoted by certain entities, most notably the oil change industry in the United States (including car dealerships). Indeed, studies have shown more wear occurs with fresher (1000-2000 mile) oil. This is attributed to additives re-establishing themselves, TBN converging, and filters becoming more efficient. Most manufacturers recommend oil change intervals of 6,000 miles or more for modern cars. In Europe, by contrast, where the influence of oil companies has been much less, oil is typically changed only at a major service interval, between 10,000 and 15,000 miles for a modern
related resources: laboratory equipment, laboratory test equipment, used laboratory equipment
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