The Society of Automotive Engineers (SAE) has established a scale which indicates oil thickness (viscosity). SAE numbers include, for example, 10W-30, 5W-40 and sometimes just a single number such as 5W, 10W, 20, 30, 40 or 50. The "W" next to a number means the oil thickness was measured at a very cold temperature, as low as -35 degrees F, when the oil is thickest. A number without a W suffix indicates the oil thickness was measured when the oil was hot (210 degrees F).
Two numbers separated by a hyphen indicate a multiviscosity oil. Multiviscosity oils are tested at both hot and cold temperatures and are recommended for all-season use. Multiviscosity oils are able to lubricate moving parts over a wide range of temperatures. These oils contain a viscosity index improver or polymers to change the viscosity of the oil as temperature changes. A multiviscosity oil such as a 10W-40 will function like an SAE 10W oil at cold temperatures and like an SAE 40 oil when the temperatures are warm or hot. Note that a multiviscosity oil can not be made by combining individual SAE oil viscosities together. Multiviscosity oils require the addition of polymers in order to gain the ability to function differently at different temperatures. Equipment operator's and service manuals specify which oil viscosity should be used under specific weather conditions.
