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Viscosity is an internal property of a fluid that offers resistance to flow. It is also commonly perceived as "thickness" of the fluid . Viscosity may be thought of as a measure of fluid friction. For example water is "thin", having a lower viscosity, while lubricating oil is "thick" having a higher viscosity.

Kinematic viscosity is the ratio of the viscosity of a fluid to its density. It is a measure of the resistive flow of a fluid under the influence of gravity. It is frequently measured using a device called a capillary viscometer -- basically a graduated can with a narrow tube at the bottom. The most common unit is probably the square millimeter per second [mm2/s] or centistokes [cSt].

The viscosity index (V.I) of oil is a number that indicates the effect of temperature changes on the viscosity of the oil.  A low V.I signifies a relatively large change of viscosity with changes of temperature.  In other words, the oil becomes extremely thin at high temperatures and extremely thick at low temperatures. On the other hand, a high V.I  signifies  relatively  little  change  in viscosity  over  a  wide  temperature  range.

The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. At this temperature the vapor may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined as the temperature at which the vapor continues to burn after being ignited.

The pour point of a liquid is the lowest temperature at which it will pour or flow under prescribed conditions. It is a rough indication of the lowest temperature at which oil is readily pumpable.Also, the pour point can be defined as the minimum temperature of a liquid, particularly a lubricant, after which, on decreasing the temperature, the liquid ceases to flow.

The Total Base Number of oil is the measure of the alkaline reserve, or the ability of the oil to neutralize acids, from combustion. TBN can also be defined as the quantity of acid, expressed in terms of the equivalent number of milligrams (mg) of Potassium Hydroxide that is required to titrate the strong base constituents present in 1 gm. of oil sample.

Additive is an agent used for imparting new, or for improving existing characteristics of lubricating oils or greases.

API Classification System is the classifications and designations for Engine oils, developed by API in conjunction with SAE and ASTM.The API service classes have two general classifications: S for "Service" (orgininating from Spark ignition) (typical passenger cars and light trucks using gasoline engines), and C for "Commercial" (originating from Compression ignition) (typical diesel equipment).Gear oils are classified by the American Petroleum Institute using GL ratings. For example, most modern gearboxes require a GL4 oil, and separate differentials (where fitted) require a GL5 oil.

The Society of Automotive Engineers (SAE) is an engineering society founded to develop, collect, and disseminate knowledge of mobility technology.

Ash content is the percent by weight of residue left after combustion of an oil sample.

International Standards Organization is an organization which is worldwide in scope, sets standards and classifications for lubricants. An example is the ISO viscosity grade system.

Monograde (single grade) is a term used to describe an oil when its viscosity falls within the limits specified for a single SAE number.

Multigrade is a term used to describe an oil for which the viscosity/temperature characteristics are such that its low temperature and high temperature viscosities fall within the limits of two different SAE numbers.

National Lubricating Grease Institute numbers are based on the ASTM cone penetration number. The grades are in order of increasing consistency (hardness).

Penetration is the consistency of a lubricating grease, expressed as the distance in millimeters that a standard needle or cone penetrates vertically into a sample of the material under known conditions of loading, time and temperature.

Worked Penetration is the penetration of a sample of lubricating grease immediately after it has been brought to 77ºF and then subjected to 60 stokes in a standard grease worker.

Drop Point is the temperature at which the grease passes from a semisolid to a liquid state. This change in state is typical of greases containing conventional soap thickeners. Greases containing thickeners other than conventional soaps may, without change in state, separate Oil.

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