From an Engineer's point of viewWednesday, July 21, 2010Valvoline is a great oil,,, but I'm not fond of STP and prefer DuPont teflon gas treatment in my hi-perf engines instead,,,Here is the run down on multi viscosity oil,,, for interest sake, if nothing else.Multi viscosity oils work like this,,, Polymers are added to a light base(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. The result is that at 100 degrees C (212 Fahrenheit),,, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-viscosity 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,,, in the summer,,, the highest temperature you expect. The polymers can "shear" and "burn" forming 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 high polymer content. IT IS THE OIL THAT LUBRICATES,,, not the additives. Oils that can do their job with the fewest additives are the best. Very few manufacturers recommend 10W-40 any more, and some threaten to void warranties if it is used. 20W-50 is the same 30 point spread,,, but because it starts with a heavier base, it requires less viscosity index improvers (polymers) to do the job. AMSOIL can formulate their 10W-30 and 15W-40 with no viscosity index improvers but uses some in the 10W-40 and 5W-30. Other multigrade synthetics may not use VI improvers either. The full literature that's available from oil companies, should include this information. Follow your manufacturer's recommendations as to which weights are appropriate for your vehicle. Viscosity Index is an empirical number indicating the rate of change in viscosity of an 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. This is one major property of an oil, that keeps your bearings "happy"... These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown. Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendancy 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. 400 F is the minimum to prevent possible high consumption. Flash point is in degrees F. Pour point is 5 degrees F 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. A borderline pumping temperature is given by some manufacturers. This is the temperature at which the oil will pump and maintain adequate oil pressure. This was not given by a lot of the manufacturers,,, but seems to be about 20 degrees F above the pour point. The lower the pour point the better. Pour point is in degrees F. % sulfated ash is how much solid material is left when the oil is reacted with sulfuric acid and burned. This is used to quantify the amount of metallic antiwear and detergent additives in the oil. Zinc dithiophosphate is a common detergent and antiwear additive that is reflected in this test. % zinc is the amount of zinc used as an extreme pressure, anti-wear additive... The zinc is only used when there is actual metal to metal contact in the engine. Hopefully the oil will do its job and this will rarely occur, but if it does, the zinc compounds react with the metal to prevent scuffing and wear. A level of .11% is enough to protect an automobile engine for the extended oil drain interval, under normal use. Those with high revving, blower or turbo charged cars or inboard boats,,, might want to look at the oils with the higher zinc content. More doesn't give you better protection,,, BUT, it gives you longer protection if the rate of metal to metal contact is abnormally high. Just,,, Food for thought.
When The Flag Drops.,. The Bull ***t Stops.,. P. Engineer, Engine Builder
Hello tuffnuff;Great info, just for clarification, are the comments below part of what you are saying?1. The first number on multi viscosity oils is the actual "viscosity" of the oil.2. When "cold", a multi viscosity oil flows at the same rate a single viscosity oil of the same viscosity would.3. When warm, multi viscosity oils do not thin as much from their stated viscosity as a single viscosity oil does.