The SAE designation on motor oil refers to the viscosity standards set by the Society of Automotive Engineers, a group of over 84,000 engineers, businessmen, educators and students who write standards for the aerospace and automotive industries. The SAE viscosity standards have changed since their inception in 1911, as research and development of engines and motor oils have advanced.
The Society of Automotive Engineers was formed in 1905 to consolidate the technical information and knowledge of the newly emerging automobile industry. Henry Ford served as the society's first vice-president. The society members wanted a system that reflected the suitability of an oil as an engine lubricant that would be easy for the consumer to understand. In 1911, the SAE published its first list of oil standards, called the SAE J300, that defined five different numbered grades for motor oil: 10, 20, 30, 40 and 50. These numbers reflected the flow rate, or viscosity, of the oil.
Simply put, viscosity is the physical property of a liquid or gas that reflects its tendency to flow. We refer to thick fluids as "high viscosity," and thinner fluids as "low viscosity." This is an important property in motor oil, because the viscosity of the oil will affect its ability to lubricate the moving parts of an engine. If the oil is too thin, the oil pump cannot maintain enough pressure to circulate it properly, and if it is too thick, the oil will not penetrate between the small working parts of the engine. Because both scenarios will cause damage to the engine, it is important that the oil be of the correct viscosity. Low viscosity oils will have an SAE rating of 5, 10 15, and 20 and flow better at lower temperatures. High viscosity oils have numbers of 30, 40 or even 50, and flow better at higher temperatures. These numbers reflect single weight oils, and are no longer used in modern automobiles.
The oil weights, as reflected in the 10, 20, 30, 40 and 50 SAE designation, were developed by finding the amount of oil that would flow through a viscometer at 210 degrees in a certain number of seconds. For example, a 20-weight oil will flow through the viscometer at a rate of 15 to 24 seconds. The Society of Automotive Engineers publishes oil weight charts that are available to view on their website.
The SAE developed the original viscosity number system based on oil flow rates tested at 98.9 degrees Celsius. A major change came in 1952, with the addition of a set of winter ("W") grade designations, based on viscosity measured at -17.8 degrees C. This change was made to address the problems with cold-weather oil performance. Pennsylvania crude oils were found to perform better in cold conditions than Gulf oils, and became the first "W" designated oils.
The original, single-weight oils are no longer used in late-model automobile engines. Multi-viscosity oils are a blend of high and low viscosity oils, or oils to which special polymers have been added, and have been developed to function in climates where there can be variations in temperature ranges. The number system for these blends reflect the oil's cold temperature and warm temperature viscosity rating. For example, in oil rated SAE 10W-40, the 10W is the oil's low temperature rating, and the 40 reflects the high-temperature rating.