Most applications of lube oils require that they be nonresinous, pale-colored, odorless, and oxidation-resistant. Over a dozen physical and chemical tests are used to classify and determine the grade of lubricating oils. Common physical tests include measurements for viscosity, specific gravity, and color, while typical chemical tests include those for flash and fire points.
Of all the properties, viscosity, a lube oil’s resistance to flow at specific temperatures and pressures, is probably the single most important one. The application and operating temperature range are key factors in determining the proper viscosity for an oil. For example, if the oil is too viscous, it offers too much resistance to the metal parts moving against each other. On the other hand, if it not viscous enough, it will be squeezed out from between the mating surfaces and will not be able to lubricate them sufficiently. The Saybolt Standard Universal Viscometer is the standard instrument for determining viscosity of petroleum lubricants between 70 and 210 degrees Fahrenheit (21 and 99 degrees Celsius). Viscosity is measured in the Say bolt Universal second, which is the time in seconds required for 50 milliliters of oil to empty out of a Saybolt viscometer cup through a calibrated tube orifice at a given temperature.
The specific gravity of an oil depends on the refining method and the types of additives present, such as lead, which gives the lube oil the ability to resist extreme mating surface pressure and cold temperatures. The lube oil’s color indicates the uniformity of a particular grade or brand. The oil’s flash and fire points vary with the crude oil’s origin. The flash point is the temperature to which an oil has to be heated until sufficient flammable vapor is driven off so that it will flash when brought into contact with a flame. The fire point is the higher temperature at which the oil vapor will continue to burn when ignited.
Common engine oils are classified by viscosity and performance according to specifications established by the Society of Automotive Engineers (SAE). Performance factors include wear prevention, oil sludge deposit formation, and oil thickening.