| [GMCnet] More on Synthetic Oils [message #106937] |
Sun, 28 November 2010 11:40 |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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If anyone wants to read more on synthetic oils go to an article that I wrote for GMC western States almost ten years ago.
http://www.gmcws.org/techcenter/01-08tc34.html
Here is an excerpt from that article:
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Multi-viscosity
Multi viscosity oils work like this: Polymers, called VI (viscosity index) Improvers, are added to a light base oil (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. I often suggest the image of trying to pour a can of peas and a can of spaghetti. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.
Multi viscosity oils are one of the great improvements in oils, but they should be chosen wisely. Always use a multi grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter base your decision on the lowest temperature you will encounter and in the summer the highest temperature you expect. The polymers can shear and burn and form deposits that can cause ring sticking and other problems. 10W-40 and 5W-30 require a lot of polymers (synthetics excluded) to achieve that range. This has caused problems in diesel engines, but fewer polymers are better for all engines. The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the higher polymer content. It is the oil that lubricates not the additives. Oils that can do their job with the fewest additives are the best. One of the problems with multi viscosity oils is that, over time, the polymers can shear and your oil will
thin down. This is another reason to change your oil at regular intervals.
Very few manufactures recommend 10W-40 any more, and some threaten to void warranties if it is used. 20W-50 has the same 30 point spread, but because it starts with a heavier base, it requires less VI improvers (polymers) to do the job. AMSOIL can formulate their 10W-30 and 15W-40 with no viscosity index improvers but uses some in their 10W-40 and 5W-30. Mobil 1 uses no viscosity improvers in their 5W-30 and in their 10W-30. I believe they use a small amount in the 15W-50 oil. Follow your manufacturer's recommendations as to which weights are appropriate for your vehicle.
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NOTE that true synthetics DO NOT use much if any of the polymers to achieve their multi-vis properties.
Emery Stora
77 Kingsley
Santa Fe, NM
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