ANSWERS: 6
  • Both oils you mentioned are called dual viscosity oils or lubricants. The first number represents the viscosity "weight" when the oil is heated to operating temperature. The second number represents the viscosity of the oil when cold. The rule of thumb is that the lower the number, the thinner the oil will be and thicker when the number is higher. The W stands for "weight". What you are seeing is an oil that exhibits different resistances to flow at different temperatures. Generally, you would use a higher weight oil for performance purposes because a more dense oil film between bearing surfaces will tend to handle bearing pressure better than an oil that is less viscous. The drawback is that a thicker oil does not flow as well and you need a lubrication system that can flow a higher volume of a thicker oil to prevent the lubricant from breaking down prematurely. You can also select an oil range for the climate you are driving in. I normally use a 10W-40, but I change to a 10W-30 for the winter because its cold viscosity is less and allows the car to crank over easily when it is darn cold out.
  • Contrary to the other answer, the lower number is the cold viscosity; "W" stands for "Winter", not "Weight". Through the wonders of chemistry, these oils actually get THICKER as they heat up. A 5W-40 might be used in the winter, and a 10W-50 in the summer. I think the two numbers are measured at 0 and 100 degrees C, respectivley. That is, at freezing temperatures, for purposes of cranking the engine, the 5W will flow easier than the 10W, making cranking easier, and getting the lubricant flowing more quickly, but sacrificing some lubrication capacity temporarliy. When the engine warms up, the 40 will flow more easily than the 50, and so may help gas mileage, but may have a little more trouble filling gaps where parts are a little loose. And when the engine runs hotter than usual, like mid-summer, the 50 will lose its viscosity more slowly than the 40; eventually heat overcomes chemistry and the oil does get thinner. From an article on the Advance Auto Parts website: http://www.advanceautoparts.com/english/youcan/html/ccr/ccr20010101ov.html "Multigrade oils typically begin as base oils, such as 10W. Then viscosity-index modifiers (polymers) are added in an effort to stabilize the viscosity. This allows a 10W40 oil to flow like a 10W at cold temperatures and a 40W at higher temperatures." The article also points out that this property is referred to a "multi-grade". "Multi-viscosity" actually has to do with a different property.
  • Geeze, all the previous answer are way too complicated. Make your life simple and check: http://en.wikipedia.org/wiki/Engine_oil#Multi-grade_motor_oil Here's an excerpt: The API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. The first number associated with the W is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as cold as a single-grade 10 weight oil can be pumped. "5W" can be pumped at a lower temperature than "10W". The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle.
  • As the others, both are multi-grade. The numbers is actually average time in seconds that a certain amount of oil will flow through a standard device when the oil is at a temperature of 100 deg C. "W" stands for winter and was added to the SAE standard in 1956. Cold oil flows slowly, hot oil flows faster. 5W means that the oil, when cold, will flow as fast as a SAE 5 single-grade oil at that same temperature (cold). The "40" means that when the oil is heated up to 100 deg C, it will flow at the same rate as an SAE 40 single-grade oil. The oil viscosity characteristics change with temperature from a SAE 5 single-grade oil to a SAE 40 single-grade oil. In a nutshell: 5W-40 is "thinner" (flows easier) than 10W-50 when hot and cold. Use 5W-50 in cold climates (winter). Use 10W-50 in warmer climates (summer).
  • Then how do you rate straight weight oil such as straight 100?
  • Straight weight oil, also known as single-grade oil is rated "SAE xxx", e.g. a 100 "weight" oil would be "SAE 100". A single-grade oil's viscosity characteristics does not change with temperature. The characteristics of a multi-grade oil, on the other hand, will change with temperature.

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