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The weights given on oils are arbitrary numbers defined by the Society of Automotive Engineers (SAE). These numbers correspond to "real" viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated as 5, 10, 20, 30, 40, or 50 weight by the SAE. A capital letter W following a range indicates that the oil meets the specifications for viscosity at 0° F (-18° C) and is suitable for winter use.
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| SAE Gear Viscosity Number |
| ________________________________________________________ |
| |75W |80W |85W| 90 | 140 | |
| |____|_____|___|______________|________________________| |
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| SAE Crank Case Viscosity Number |
| ____________________________ |
| |10| 20 | 30 | 40 | 50 | |
| |__|_____|____|_____|______| |
______________________________________________________________
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
viscosity cSt @ 100 degrees C
- Viscosity Index: a number indicating the rate of change in viscosity of oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change. The higher the number the better. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.
- Film strength: the amount of pressure required to force out a film of oil from between two pieces of flat metal. The higher the film strength, the more protection is provided wherever the lubricant is not under oil-system pressure.
- Flash point: the temperature at which oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. The flash point can be an indicator of the quality of the base stock used. The higher the flash point the better. A temperature of 400° F (204° C) is the minimum to prevent possible high consumption.
- Pour point: a value 5° F (3° C) above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. The lower the pour point temperature, the better.
- Percent sulfated ash: a value indicating the amount of solid material remaining when the oil burns. A high ash content will tend to form more sludge and deposits in the engine. Low ash content also seems to promote long valve life. Therefore, a low ash content is desirable.
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