Q. What do the
numbers and letters mean and how do I use them to find the right viscosity
grade for my engine?
Viscosity is the measure of how thick an oil is. This is the most important property for an engine. It is a measure of the oil's resistance to flow. the more resistant or "thick" the oil, the higher its viscosity. An oil with too low a viscosity can shear and loose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.
A system for rating the viscosity of oils was established many years ago by the SAE. An oil's viscosity rating is often referred to as its "grade" (or from times past, its 'weight'). An oil that flows more quickly has a lower vscosity and, consequently, is given a lower rating. Grade numbers are assigned to certain ranges of viscosity ratings. For example, an SAE 30 grade covers a lower range of viscosities than an SAE 40 grade.
Multigrade oils, such as a 10w30 oil, are formulated to flow rapidly to areas requiring lubrication when the engine is cold but also to maintain enough viscosity to protect the engine at higher temperatures and operating loads. The number before the 'W' (for Winter ) is the oil's viscosity when cold. The number after the 'W' is its viscosity at operating temperatures.
We recommend following the engine manufacturer's suggestions for viscosity grade when choosing an oil for normal driving conditions. Particularly, keep in mind that when driving in cold climates, an oil with good start-up and flow behavior is preferred, such as a 5w30 or 10w30. An older, more worn engine may be better protected with a higher viscosity oil, like a 10w40 or 20w50. Engines that operate in a warm climate may only require a higher viscosity, single grade engine oil, SAE 30 or 40 for example. Turbo-charged and high-performance engines may need an oil that lubricates well at start-up but also continues to protect with the engine is hot, perhaps a 15w40 or even 20w50 in warmer weather.
Viscosity Index or VI is a measure
of how much an oil's viscosity changes with temperature changes. The higher
the VI, the less change that occurs and the better the protection.
Starting Viscosity of and oil determines how easily the oil will allow a cold engine to turn over and start at lower temperatures. The lower the viscosity and SAE 'W' grade at that temperature, the easier the engine will start. This is particularly true with diesel engines.
Modern gasoline engines with electronic ignition and fuel injection start more easily at low temperatures, making starting viscosity less of a concern. However, this leads to another more critical concern- the engine starts but the oil cannot be adequately pumped. Ths is descussed further in the section 'Pumping Viscosity'.
To measure the starting viscosity of an oil, ASTM test method D5293 is used. In this test, a steel cylinder, or 'rotor', is inserted into a relatively close-fitting copper cup, or 'stator', filled with the oil in question.
The oil is cooled to a specific temperature at which point a certain amount of engery is applied to the rotor to turn it against the resistance of the oil. The rotor speed is measured to determine the engine oil viscosity at the shose temperature. The unit of measurement for reporting starting viscosity is centipoise(cP).
SAE has set specifications for starting viscosity associated with the 'W' grade of an oil as follows:
Viscosity Grade Maximum Starting Viscosity
6000 cP at - 5*C (+23*F)
starting my engine in colder weather, why should I watch my oil presure
Pumping Viscosity Once an engine has been started, it needs a continuous and adequate supply of oil. The pumping viscosity of an oil determines how easily an oil will flow from the oil pump to critical parts of the engine in cold temperatures.
There are two types of pumpability problems. If the oil is not sufficiently pumpable (a condition known as being flow-limited) the oil may flow but not rapidly enough to prevent progressive damage to the areas needing lubrication during low temperature starting and operation. If the oil is not pumpable at all at lower temperatures (a condition known as air-binding or gelation) the engine will be severly damaged in a relatively few minutes. This condition is discussed further in the Gelation Index section.
A flow-limited oi may not be adequately pumped because of having too high a viscosity at the low temperature at which the engine is started. To measure the pumping viscosity of an oil, ASTM test method D4684 is used. In this Test, a rotor is inserted into a stator containing the oil in question and very slowly cooled at a constant rate over approximately a two-day period until it reaches a specific temperature. A samll force is then applied to turn the rotor and the rate at which the rotor turns is related to the viscosity. The unit of measurement used for reporting pumping viscosity is centipoise (cP).
As of january, 1996, SAE revised the specifications for pumping viscosity associated with the 'W' grade of oil. The new spcifications are as follows:
Viscosity Grade Maximum Starting Viscosity
60,000 cP at -15*C (+5*F)
Q. I've heard that oil can turn into a thick, jelly-like substance in the crankcase. Why does this happen and what does it do to the engine?
Gelation index Of the two forms of pumpability problems, the air-binding or gelation condition is of most concern. This is when the engine oil forms a gel in the crankcase under particular slow cooling conditions -- cooling conditions that are not necessarily very low in temperature. Under these conditions, the engine can be started relatively easily. Depending on the severity of gel formation, air-binding can result in which, in an effort to pump the gelled oil, a hole forms from the surface of the oil in the crankcase to the oil pump inlet and the pump draws air rather than oil. Fortunately, the cooling cycle producing this condition is not very common although some oil formulations are more susceptible than others. In extreme cases of gelation, the oil can actually become solid.
The Gelation Index is a number indicating the oil's tendency to form a gelated structure in the oil at colder temperatures. Numbers above 6 indicate some gelation-forming tendencies. Numbers above 12 are of concern to engine makers. Numbers above 15 have been associated with field-failing oils.
An oil's Gelation Index is determined by using ASTM test method D5133. A tube of oil containing a rotor driven at 0.3 RPM is slowly cooled at 1°C per hour for approximately two days. This technique allows the viscosity and any gelating tendency to be continuously measured. Data is collected throughout the cooling cycle and then analyzed to determine whether gelation is present and how severe it is. The Gelation Index has no assigned unit of measurement.
Specifications set by ILSAC (International Lubricant Standardization and Approval Committee) determine that the gelation index of an oil should be 12 or less when evaluated in the temperature range of -5°C (+23°F) to -40°C (-40°F).
Q. How does oil affect my engine's durability as well as its fuel economy?
Operating viscosity Of the different types of viscosity measurements, an oil's operating viscosity is the most important. It is this property which permits an engine to operate at all since it determines the oil's ability to lubricate critical areas of the engine such as the bearings, where temperatures can be greater than 300°F and the piston rings where temperatures can be much higher.
A sufficiently high viscosity at high operating temperatures protects the engine parts from damaging contact with one another. However, even though higher operating viscosity is more protective, it also takes more energy to move the engine parts, resulting in poorer fuel economy.
Oil experiences very high stress while in use, causing it to lose viscosity as a result of shearing. An oil's resistance to viscosity loss during operation directly affects its ability to minimize engine wear. This is discussed more in the section titled 'Shear Stability'.
With the advent of multigrade oils, it has become necessary to measure the operating viscosity nearer the high flow rates encountered between the moving surfaces in an engine.
Thus, the operating viscosity of an oil is measured using ASTM test method D4683. In this test, the oil is placed in a stator with a very close-fitting rotor and brought to the test temperature of 150°C (30°F). The rotor is then forced to turn at 3600 RPM while only 3.5 microns (1/30th of a human hair) from the stator. The force required to turn the rotor at this speed and gap is measured and translated to viscosity in units of centipoise.
The SAE's engine oil classification system sets the following values for engine oils according to their high temperature, high flow rate viscosities:
Minimum Operating Viscosity
Q. What causes the sludge that is sometimes found in the bottom of the oil pan?
Oxidation resistance Oxidation resistance is the ability of an oil to resist the direct and indirect attack of oxygen during engine operation. The way in which an oil is formulated determines its ability to resist oxidation. Strong oxidation begins to occur rapidly after the antioxidant additive in the engine oil is exhausted, so the type and amount of antioxidant in the oil determines how long oxidation will be resisted.
Oxidation leads to the formation of acid, deposits, and varnish within the engine which can cause stuck piston rings, scored cylinders, and other engine damage. Oil viscosity can increase with oxidation and, in extreme conditions or with poor engine oil, may cause the oil to gel in the crankcase at normal ambient temperatures
A much faster way of measuring oxidation resistance than in an engine test is by using ASTM test method D4742. In this test, the time for an oil to reach its 'break point' measures when the oil's oxidation resistance is overcome. When this happens, the oxygen pressure suddenly drops as a result of oxygen chemically attacking the engine oil and forming oxidation products. The time it takes for the breakpoint to occur is recorded in minutes.
There are no specifications for the length of time an oil must perform
before showing signs of oxidizing. However, a longer time to reach the
breakpoint in this test indicates an engine oil may be more resistant to
oxidation -- a desirable property
There are several concerns associated with oil volatility. First, engine oil is lost and must be replaced. Additionally, the viscosity of the remaining oil increases. Moreover, oil loss could change the effectiveness of the oil as a lubricant. The chemical properties and additive package ratio of the oil may also change as the volatile components are burned off. Finally, there is also evidence that the volatilized oil may damage the exhaust catalyst as it passes through.
The volatility of an engine oil is measured using ASTM test method D5800. A known weight of oil is heated to 250°C in a special chamber and held at that temperature for one hour. Air is introduced into the chamber and maintained at a constant flow rate under slight vacuum. After one hour, the amount of oil remaining in the chamber is weighed again. The percentage of oil lost is determined by comparing the remaining weight of oil with the original weight of oil.
Specifications set by ILSAC determine that the amount of engine oil
lost through volatilization at 250°C for one hour should not exceed
Shear stability Shear stability is a measure of the amount of viscosity an oil may lose during operation. Oil experiences very high stresses in certain areas of the engine such as in the oil pump and cam shaft area. Most multigrade engine oils contain special types of additives, called Viscosity Index Improvers, which are composed of very large, viscosity-controlling molecules. As the oil passes through the engine, these molecules are permanently sheared or torn apart over time, causing the additive to lose some of its viscosity-contributing advantages.
Oils that do not contain special Viscosity Index Improver additives,
such as single grade oils, will experience little or no permanent viscosity
Although there are no specifications indicating required levels of shear
stability for engine oils, lower percentages mean that an oil is more shear
stable and will retain its viscosity better during operation
acid resistance - Acid resistance is a measure of an oil's ability to neutralize harmful acids in the engine. During operation, the engine produces fuel combustion by-products which form acids. As engine oil oxidizes, it also forms acids. Unless neutralized, these acids can damage an engine by attacking engine components.
Engine oils are formulated to contain components called 'bases' which counteract and neutralize acids. An oil's level of acid resistance is determined by measuring the amount of base components in the oil. This is done by using ASTM test method D2896. The oil is mixed with increasing amounts of an acid solution until the base components are overcome. The amount of base components that were present in the oil can then be calculated to determine the Total Base Number (TBN), expressed as milligrams per gram of oil (mg/g).
No specifications have been set for acceptable TBN limits. Up to a point, the higher the TBN number, the more ability an oil has to protect the engine from acids. If the TBN number is too high for the particular engine type or design, however, excessive ash may form during oil combustion which could cause scouring and wear of the cylinder walls. In general, an average range for TBN in gasoline engine oils ranges from 6 to 12 mg/g. TBN numbers below 6 mg/g signal a need to change the oil more frequently to prevent acid buildup.
TBN measurements are frequently used for fleet monitoring to determine the amount of change in the oil as it is used in the engine and to predict the useful remaining life of the engine oil.
Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. The flash point can be an indicator of the quality of the base stock used. The higher the flash point the better. 400 F is the minimum to prevent possible high consumption. Flash point is in degrees F.
Pour point is 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. A borderline pumping temperature is given by some manufacturers. This is the temperature at which the oil will pump and maintain adequate oil pressure. This was not given by a lot of the manufacturers, but seems to be about 20 degrees F above the pour point. The lower the pour point the better. Pour point is in degrees F.
% sulfated ash is how much solid material is left when the oil burns. A high ash content will tend to form more sludge and deposits in the engine. Low ash content also seems to promote long valve life. Look for oils with a low ash content.
% zinc is the amount of zinc used as an
extreme pressure, anti- wear additive. The zinc is only used when there
is actual metal to metal contact in the engine. Hopefully the oil will
do its job and this will rarely occur, but if it does, the zinc compounds
react with the metal to prevent scuffing and wear. A level of .11% is enough
to protect an automobile engine for the extended oil drain interval, under
normal use. Those of you with high revving, air cooled motorcycles or turbo
charged cars or bikes might want to look at the oils with the higher zinc
content. More doesn't give you better protection, it gives you longer protection
if the rate of metal to metal contact is abnormally high. High zinc content
can lead to deposit formation and plug fouling.
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