SAE viscosity number -- system for classifying crankcase, transmission, and differential lubricants, according to their viscosities, established by the Society of Automotive Engineers. SAE numbers are used in connection with recommendations for crankcase oils to meet various design, service, and temperature requirements affecting viscosity only; they do not denote quality.

Saturation level -- the amount of water that can dissolve in a fluid.

Saybolt Universal Viscosity (SUV) or Saybolt Universal Seconds, (SUS) -- the time in seconds required for 60 cubic centimeters of a fluid to flow through the orifice of the Standard Saybolt Universal Viscometer at a given temperature under specified conditions. (ASTM Designation D 88.)

Scuffing -- abnormal engine wear due to localized welding and fracture. It can be prevented through the use of antiwear, extreme-pressure and friction modifier additives.

Semisolid -- any substance having the attributes of both a solid and a liquid. Similar to semiliquid but being more closely related to a solid than a liquid. More generally, any substance in which the force required to produce a deformation depends both on the magnitude and on the rate of the deformation.

Shear rate -- rate at which adjacent layers of fluid move with respect to each other, usually expressed as reciprocal seconds.

Shear stress -- frictional force overcome in sliding one "layer" of fluid along another, as in any fluid flow. The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear. In a non-Newtonian fluid -- such as a grease or a polymer-containing oil (e.g. multi-grade oil) -- shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an apparent viscosity, a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined.

Silt -- contaminant particles 5 µm and less in size.

Silting -- a failure generally associated with a valve which movements are restricted due to small particles that have wedged in between critical clearances (e.g., the spool and bore.)

Single-pass test -- filter performance tests in which contaminant which passes through a test filter is not allowed to recirculate back to the test filter.

Sintered medium -- a metallic or nonmetallic filter medium processed to cause diffusion bonds at all contacting points.

Sleeve bearing -- a journal bearing, usually a full journal bearing.

Sludge -- insoluble material formed as a result either of deterioration reactions in an oil or of contamination of an oil, or both.

Soap -- general term denoting the salt of a fatty acid. The ordinary soaps are those of sodium and potassium. The soaps of lithium, calcium, sodium, and aluminum are the principal thickeners used in grease making.

Solid -- any substance having a definite shape which it does not readily relinquish. More generally, any substance in which the force required to produce a deformation depends upon the magnitude of the deformation rather than upon the rate of deformation.

Solvency -- ability of a fluid to dissolve inorganic materials and polymers, which is a function of aromaticity.

Spall – to break up or damage a surface by moderate to deep cracking or flaking.

Specific gravity -- the ratio of the weight of a given volume of material to the weight of an equal volume of water.

Spectrometry – using the analysis of electromagnetic radiation (light) to determine trace elements and their concentrations in a sample. In the atomic spectrometry techniques most commonly used for trace element analysis, the sample is decomposed by intense heat into a cloud of hot gases containing free atoms and ions of the element of interest. In general, there are four types of thermal sources normally used in analytical atomic spectrometry to perform this decomposition process: flames, furnaces, direct electrical discharges, and plasmas. Instruments which separate, isolate and measure light by wavelength in this way are called spectrographs or spectrometers.

Atomic absorption spectrometry shines a light of a wavelength characteristic of the element of interest through the hot vapor (usually created by a flame or furnace). Some of this light is then absorbed by the atoms of that element. The amount of light that is absorbed is measured and used to determine the concentration of that element in the sample.

Optical emission spectrometry uses electrical discharges or radio-frequency stimulated plasmas to bring the sample to temperatures high enough to dissassociate the sample into atoms and introduce significant amounts of excitation and ionization through atom-to-atom collisions. Once the atoms or ions are in this excited state, then they decay to lower states by radiating light. The intensity of the light is measured at specific wavelengths and used to determine the elemental concentrations. For example, two popular types of spectrometers use the emission principle:

Rotating disc emission (RDE) spectroscopy, where the electric arc struck between a rod and a rotating disc or between two rotating discs provides the energy source;

Inductively coupled plasma (ICP) spectroscopy, where a high-temperature discharge is generated by flowing a conductive gas (for example, argon) through the magnetic field generated by a radio-frequency load coil that surrounds the tubes carrying the gas. This highly energetic source stimulates the necessary light emission from the elements being analyzed.

Spin-on filter -- a throw-away type bowl and element assembly that mates with a permanently installed head.

Spindle oil -- a light-bodied oil used principally for lubricating textile spindles and for light, high-speed machinery.

Splash lubrication -- a system of lubrication in which parts of a mechanism dip into and splash the lubricant onto themselves and/or other parts of the mechanism.

Stainless steel – any of several steels containing 12% to 30% chromium as the principal alloying element; the steels usually are passive in aqueous environments.

Starting fluid (diesel) -- a fluid, such as diethyl ether, which has a wide flammability range; used to start diesel engines at extremely low temperatures.

Static friction -- the force just sufficient to initiate relative motion between two bodies under load. The value of the static friction at the instant relative motion begins is termed break-away friction.

Statistical process control – use of control charts and appropriate procedures to track and eliminate unacceptable physical variables in repetitive manufacturing processes.

Statistical quality control -- use of control charts and appropriate procedures to demonstrate consistency in the final quality of a product.

Stoke (St) -- standard metric kinematic unit of measurement of a fluid's resistance to flow, defined by the ratio of the fluid's dynamic viscosity to its density, equal to 1 cm² s^-1. The standard reporting unit of measurement for kinematic viscosity is the centistoke, equal to 1/100 of a stoke.

Strainer -- a coarse filter element (pore size over approximately 40 µm).

Stringer – wear analysis-specific term for a stringlike or striplike formation of small joined flakes, usually associated with groups of small platelets which have not separated into individual particles.

Suction filter -- a pump intake-line filter in which the fluid is below atmospheric pressure.

Sulfurized oil -- oil to which sulfur or sulfur compounds have been added.

Surface filtration -- filtration which primarily retains contaminant on the influent surface.

Surface tension -- the contractile surface force of a liquid by which it tends to assume a spherical form and to present the least possible surface. It is expressed in dynes/cm or ergs/cm2.

Surfactant -- surface-active agent that reduces interfacial tension of a liquid. A surfactant used in a petroleum oil may increase the oil's affinity for metals and other materials.

Surge -- a momentary rise of pressure in a circuit.

Swarf -- the cuttings, and grinding fines that result from metal working operations.

Switch, pressure -- an electric switch operated by fluid pressure.

Synthetic lubricant -- a lubricant produced by chemical synthesis rather than by extraction or refinement of petroleum to produce a compound with planned and predictable properties.

Sulfated ash -- the ash content of fresh, compounded lubricating oil as determined by ASTM Method D 874. Indicates level of metallic additives in the oil.

Synthetic oil -- lubricant produced by synthesis rather than by extraction or refinement.