Multi-grade
The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather. A specific oil will have high viscosity when cold and a low viscosity at the engine's operating temperature. The difference in viscosities for any single-grade oil is too large between the extremes of temperature. To bring the difference in viscosities closer together, special
polymer additives called
viscosity index improvers, or VIIs are added to the oil. These additives make the oil a
multi-grade motor oil. The idea is to cause the multi-grade oil to have the viscosity of the base number when cold and the viscosity of second number when hot. This enables one type of oil to be generally used all year, and when multi-grades were initially developed, they were frequently described as
all-season oil. The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened. This slope representing the change with temperature depends on the nature and amount of the additives to the base oil.
The SAE designation for multi-grade oils includes two grade numbers; for example,
10W-30 designates a common multi-grade oil. Historically, the first number associated with the
W (again 'W' is for Winter, not Weight) is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as well as a single-grade SAE 10 oil can be pumped. "5W" can be pumped at a lower temperature than "10W" and "0W" can be pumped at a lower temperature than "5W". The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. This "classic" method of defining the "W" rating has since been replaced with a more technical test where a "cold crank simulator" is used at increasingly lowered temps. A 0W oil is tested at −35 °C (−31 °F), a 5W at −30 °C (−22 °F) and a 10W is tested at −25 °C (−13 °F). The real-world ability of an oil to crank in the cold is diminished soon after put into service. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle (although some modern European cars now make no viscosity requirement), but can vary from country to country when climatic or mpg constraints come into play.
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