![]() ![]() For instance, an SAE 30 must be between 9.3 and <12.5 cSt at 100o C, that’s the approximate temperature of the bulk oil in the engine oil circulating system. Each “non-W” viscosity grade has a range of viscosities at 100o C which it must fall within. ![]() The “non-W”, high temperature viscosity grades have their viscosity measured at high temperatures representative of the conditions that the oil encounters in an operating engine. “W”/winter grades also have to meet a minimum high temperature viscosity. As well, there’s a Low Temperature Pumping Viscosity which ensures that the oil at low temperatures isn’t so thick so that the oil pump cannot move it. There’s a Low Temperature Cranking Viscosity which sets a viscosity requirement at various low temperatures to ensure that the oil isn’t too thick so that the starter motor can’t crank the engine over. The “W”/winter viscosity grades describe the oil’s viscosity under cold temperature engine starting conditions. There are the “W”/winter, grades of engine oil and the straight high temperature viscosity grades (the “non-W” viscosity grades). SAE viscosity grades are divided into two distinct groups. These days the SAE publication J300 Viscosity Grades for Engine Oils defines engine oil viscosity grades by four different viscosity tests. It was common in areas with large summer to winter temperature variations for an engine to be run on a less viscous oil grade, such as SAE 10, in winter and on a thicker SAE 30 in the summer. This first SAE Viscosity Grades for Engine Oils classification was for ‘straight’ monograde engine oils. In 1911 it established its viscosity grade classifications for engine oils. The Society of Automotive Engineers (SAE) was formed in 1905. And a multi-grade engine oil has to survive all these different conditions. It provides the appropriate fluid film protection in a number of flow and temperature conditions, being cranked at cold start-up temperatures down as low as -35o C, circulating as the bulk engine oil at around 100o C, or being torn apart through the ring belt area at temperatures around 185o C. Probably the most common multi-grade viscosity classification we encounter is an SAE 15W-40, though some modern diesel engines are now calling for SAE 10W-40, SAE 5W-30 or even SAE 0W-20 viscosity engine oils.Ī multi-grade engine oil is a technological marvel, yet we tend to take them for granted. Any lubricant, including diesel engine oils, can be fully described by those two functions, performance and viscosity. The other is performance, whether an API Service Classification, an ACEA Classification or an OEM specification. Viscosity is one of two characteristics which we use to fully describe an engine oil. We’re all familiar with multi-grade diesel engine oils. ![]()
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