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The numbers 0, 5, 10, 15 and 25 are suffixed with the letter W, designating they are "winter" (not "weight") or cold-start viscosity, at lower temperature.
originally posted by: bigfatfurrytexan
but first i've heard of it.
Viscosity (a fluid's resistance to flow) is rated at 0° F (represented by the number preceding the "W" [for Winter]) and at 212° F (represented by the second number in the viscosity designation). So 10W-30 oil has less viscosity when cold and hot than does 20W-50. Motor oil thins as it heats and thickens as it cools
The first number on a multigrade oil is normally followed by a W, which stands for winter. This number represents the lubricant’s viscosity under lower temperatures, giving an indication of how the oil will flow in the winter. The lower the first number, the thinner it is at low temperatures.
originally posted by: FlyingFox
It's 30 weight oil, that has a 5w winter rating.
It's a 30 weight oil that has a 10w winter rating, a test done at extreme cold temps.
Up until the 1960s it was quite common to change the oil depending on the season and old handbooks from the period recommend oils as low as SAE 5W for very cold ambient conditions. SAE 30 and 40 grades would be typical for Summer use, and sometimes a SAE 50 grade would be substituted if the engine started to use more oil. Multi-grade oils were developed to allow year round use as a result of artificially raising the VI of the oil.
Scientists discovered the use of polymers and the fact that these would expand with heat. VI Improvers are polymer material that expands as the oil warms up. This does NOT thicken the oil as is often stated. It merely slows down the rate at which oil thins out as the temperature rises. The base oil in use in a multi-grade (aka multi-seasonal) is the first number (such as 10W or 20W), where as the second grade is the viscosity achieved using the VI Improver (such as 40 or 50). 20W50 is a base oil of SAE 20W and the viscosity grade at running temperature is SAE 50.
The Society of Automotive Engineers (SAE) developed a scale for both engine and transmission oils. The measurement is undertaken in a laboratory in accordance with standard procedures. W is Winter and oils with the W must meet the requirements of the Cold Cranking and Cold Pumping criteria.