![]() The problem with using a lighter oil in the winter was that once an engine warmed up, the oil would get warmer and as the oil got warmer, its viscosity would decrease. The oils intended for winter use had a "W" after the viscosity rating, such as "20W." They also differed from non-winter oils in that their viscosity ratings were determined using dynamic viscosity tests that simulated their ability to be forced through an engine's many oil passages at cold temperatures. The "solution" to this problem was an imperfect one: people used lower viscosity oils in the winter. Worse yet, if it did start, it might be deprived of adequate lubrication until the oil warmed up enough to be easily pumped through the engine. On a cold winter day, SAE 30 oil might get so thick that the car's engine wouldn't even turn over. The viscosity of a monograde motor oil increases (that is, it gets thicker) as its temperature goes down. For people who lived in cold climates, not so much. That was all well and good if you happened to live someplace where it never got cold. If you bought SAE 30 oil, for example, it had a kinematic viscosity of 30 at a temperature of 100° C (212° F). They had only one viscosity rating that was based on their kinematic viscosity at 100° C (212° F). Once upon a time, all motor oils were single-viscosity or "straight weight" oils. Motor oils with higher dynamic viscosity require more force to move them through the engine so they can get to the parts they need to lubricate. The deformation can be that caused by moving a foreign body through the fluid (for example, by stirring) or it can be deformation caused by forcing the fluid through an orifice, which is more relevant to vehicle engines. Mathematically, it's the ratio of dynamic viscosity to the density of the oil.ĭynamic viscosity (or absolute viscosity) is a measure of a fluid's resistance to deformation or flow when an external force other than gravity is applied. ![]() In practical terms, it's a measure of the oil's resistance to deformation due to its own internal molecular characteristics, with no force applied other than that of gravity acting on the oil's own weight. This measure of an oil's rate of flowing through a calibrated orifice due to gravity alone is known as the oil's kinematic viscosity. The more time it takes for the cup to empty, the higher the oil's viscosity at that temperature. Traditionally, motor oil viscosity has been measured using a device called a viscometer, which is basically a cup with a calibrated hole or tube on the bottom. You can pour a glass of apple juice a lot more quickly than you could pour a glass of molasses. The higher the viscosity, the thicker the oil, and the more resistant it will be to flow. It may help to think of apple juice as a low-viscosity fluid, and molasses as a high viscosity fluid. ![]() Oils with higher viscosities are thicker and will flow more slowly. Oils with lower viscosities are thinner and will flow more easily. Simply-stated, viscosity refers to an oil's resistance to flowing at a given temperature. But what do those numbers mean in plain language? When a jug of oil says "10W-40" or "5W-30, for example, those numbers refer to the oil's viscosity. A motor oil's viscosity, sometimes called its weight, is usually the most prominent specification on the oil's label.
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