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Can someone please explain what the grade description means ? I am sure I have asked this before with no joy. If an oil is 10w40 what does the 10 and 40 mean ? If an engine recommendation is 5w40, is 10w40 suitable ?
SAE Viscosity Ratings: Viscosity refers to how "thick" or "thin" a liquid is, or how easily it pours. Viscosity also is commonly referred to as "weight," as in a light-weight or heavy-weight oil. Viscosity is really a bit more complicated than simply "thick" or "thin" or "weight," but the Society of Automotive Engineers has organized viscosity ratings in a series of numbers that is easy to understand.
Oil viscosity is affected by temperature. A heavy oil that stays relatively thick at high temperatures would have a high-viscosity rating of 30, 40, or 50. A thin oil that flows freely at low temperatures would have a lower number. Because temperature affects how well any liquid flows, motor oil viscosity is rated at both high and low temperatures. The lower viscosity numbers of 20, 15, 10, and 5 are accompanied by a "W" for "winter." Some motor oils today have a single viscosity rating, such as SAE 30, but many are designed to work in a wide range of temperatures. Such oils have a dual viscosity rating, such as 5W-20 or 15W-30.
When an oil is cold its viscosity increases, and it does not flow easily. If you use high-viscosity oil in low-temperature weather, heavily loaded engine parts will not receive oil until the engine warms and the oil thins. Hot oil, on the other hand, is thin and flows easily. Low-viscosity oil in an engine running at very high temperature may break down and allow moving parts to rub against each other. This can cause rapid engine wear and possible damage. Today, most carmakers recommend multiviscosity oils such as 5W-30 and 10W-30. Check your owners manual to see whats recommended for your car.
Modern metallurgy allows engineers to build engines with tighter clearances between moving parts than was possible in the past. These modern engine designs offer improved fuel economy, emission control, and performance, but they require motor oil that provides immediate lubrication to close-tolerance parts. High-viscosity oil may delay critical lubrication right after startup, even in hot weather. This can lead to premature engine wear and reduced operating efficiency. The best advice for selecting a motor oil that is right for your car is to follow the manufacturers recommendations for the general climate in which you drive.