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Message: <blockquote data-quote="f8." data-source="post: 135436" data-attributes="member: 923">excerpt from <a href="http://en.wikipedia.org/wiki/Motor_oil" target="_blank">wikipedia</a>:Multi-grade motor oilThe 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. 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 are added to the oil. These additives make the oil a multi-grade motor 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 API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. The first number associated with the W is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as cold as a single-grade 10 weight oil can be pumped. "5W" can be pumped at a lower temperature than "10W". 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. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle.excerpt from <a href="http://www.mobil1.com/USA-English/MotorOil/Car_Care/AskMobil/Viscosity_Designation.aspx" target="_blank">Mobil 1</a>In simple language, the first part of the viscosity designation (W grade) is an indication of the product's ability to help an engine crank and start and for the engine to pump the lubricant. The lower the number (0W is the lowest), the lower the temperature the product can be used. So the W grade is related to the lowest temperature your engine sees when you start the engine on the coldest morning of the year. But also keep in mind that a lower W grade pumps and helps an engine to start better than a higher W grade. The second part the viscosity grade is related to the viscosity your engine sees at operating temperature. In this case, a higher number is a higher viscosity grade and provides more viscous oil at operating temperature than lower viscosity grade oil. It is not safe to assume that a higher viscosity oil is always better for your engine because other factors, such as engine design, fuel economy and power, are also related to operating viscosity. You should always consult your owner's manual for the right oil to use for your particular engine. For more information on the viscosity grade specifications, a quick search of the Internet will identify several Web sites where information about SAE J300 (Viscosity Properties Test) can be found.</blockquote>

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