A lubricant is the material that lies between two surfaces that are moving with respect to each other. The presence of a lubricant affects the friction between the two surfaces. It is usually used to reduce friction, thereby reducing heat and wear, but it is also often used to cool, clean and protect the surfaces from corrosive chemical attack.
A lubricant can be liquid, solid or even a gas. Greases are liquids that have been thickened by the addition of chemical or solid materials. Teflon® and graphite are examples of solid lubricants.
Viscosity is a measure of a fluid's resistance to flow. For lubricating oil in general, viscosity is the most important physical property. It is viscosity, as well as the pressure and speed of movement, which determines the thickness of an oil film between two moving surfaces. This in turn determines the ability of the oil film to keep the two surfaces apart, the rate heat is generated by friction and the rate the oil flows between the surfaces and thus conveys the heat away.
The oil should have a viscosity at the operating temperature that is correct for maintaining a fluid film between the bearing surfaces, despite the pressure tending to squeeze it out. While a reasonable factor of safety is usually desirable, excessive viscosity should be avoided because this can create more drag and therefore unnecessary heat generation.
Viscosity is also useful for identification of grades of oil and for following the performance of oils in service. An increase in the oil’s viscosity during use usually indicates that the oil has deteriorated to some extent, a decrease normally indicates dilution with fuel. The permissible extent of viscosity increase before corrective measures are taken is largely a matter of experience and judgement of the operator.
SAE stands for the Society of Automotive Engineers, based in the USA. The SAE grade specifies the most important parameter for engine oil mainly its viscosity. In other words it tells you the "thickness" of the oil. The lower the number, the "thinner " the oil; thus SAE 30 is less viscous than SAE 40.
API stands for the American Petroleum Institute. This body has specified the performance standards that oils used in road vehicles should meet, notably for cars and trucks made in the USA. For oils destined for use in passenger car engines, the letters API are followed by a set of two letters such as SJ, etc. This indicates the Service Level for passenger car oils. These specified performance levels have evolved through the years, from API SA to SM to SN.
This refers to the origin of the base fluid. Mineral oils are derived by refining processes, essentially a complex series of purification and separation steps, from crude petroleum oil extracted from the ground. Synthetic base fluids are made by chemical processes, generally by building up larger molecules from smaller ones. Because these chemical reactions and starting materials are well defined, the synthetic fluids are not only relatively pure chemicals but are deliberately made to deliver the performance characteristics required in a lubricant.
Base Oil (sometimes also called base stock) is the name given to the main liquid component (or components) of a lubricant. It is. Base oils are mineral (or petroleum) or synthetic in origin, although vegetable oil-derived stocks may be used for specialised applications. The base stock provides the basic lubricating requirements of a lubricant.; i.e. the "oiliness"
However, in most modern lubricants a base oil mixture alone is insufficient to deliver the technical performance characteristics required and to keep the product from rapid degradation in use. Therefore the lubricant manufacturer will mix the base oils with a variety of different additives, each chosen to impart additional performance benefits to the finished oil.
Synthetic base oils are chemicals that have been made, or synthesised, by combining several smaller molecules together. There are several different types, each with its own suite of physical and chemical properties, and each ideal for a selected set of uses in lubrication. Because these are ‘made to design', and are usually quite pure in composition, the lubricants they are used in can have specific properties which cannot easily be achieved through the use of mineral base oils.
In most cases the answer is "yes". Synthetic oils are man- made lubricants which were originally created for jet aircraft engines. They have a wide range of performance and can protect engine at very high and very oil temperature conditions. In other words, they have exceptional thermal stability.
The main disadvantage of synthetic lubricants is that they tend to be more expensive than mineral oils. This restricts their use to specialty oils and greases which command premium prices. Coincidentally, oil marketers therefore ensure that their synthetic oils are also capable of the highest performance possible