Viscosity index verse viscosity number11/28/2023 Other lubricants can have VIs ranging over 400. Lubricants with VIs as low as minus 60 are available today. Better yet, you can enter a single known viscosity (and temperature) and VI (commonly found on a lubricant’s product data sheet) to calculate the viscosity of the same lubricant at any other temperature (say, a machine’s operating temperature). For instance, if you know the viscosity of a lubricant at two different temperatures, you can use the calculator to give you an estimation of the VI. You can use these calculators in a variety of ways. The temperatures used to determine the VI are 40 degrees C to 100 degrees C.Īn Internet search on “viscosity index calculator” will direct you to several Web pages. The higher the VI, the more stable the viscosity across a range of temperatures (more desirable). Halfway in between was a VI of 50, and so forth. If it was similar to Texas Gulf crude, it was assigned a VI of 0. If a lubricant was similar to the Pennsylvania crude, it was assigned a VI of 100. Based on the methodology, Pennsylvania crude (paraffinic) was set as a benchmark at one extreme, representing low viscosity changeability relative to temperature.Īt the other extreme was Texas Gulf crudes (naphthenic). It is an empirically derived, unitless number. ![]() The viscosity index was developed for this purpose (ASTM D2270) by E. Most importantly, for any candidate oil, you need to know how viscosity changes relative to a change in temperature. Remember, the ISO Viscosity Grade system only reports viscosity at a single temperature: 40 degrees C. That said, because machines require a certain viscosity, and temperature is known to have a dramatic influence on viscosity, it is imperative that you take the average operating temperature and temperature range into account when selecting viscosity. You may recall that the well-known Stribeck curve doesn’t have a temperature variable. Instead, a machine’s viscosity requirement is based on such things as component design (e.g., bearing), loads and speeds. Minimum, maximum and optimum viscosity requirements demanded by machines don’t take temperature into account. Conversely, machines don’t care about temperature as it relates to viscosity (loosely speaking). I like and have good luck with Phillips 20W-50M for break-in.However, a lubricant’s viscosity is meaningless unless the temperature is noted, i.e., the temperature at which viscosity is measured. I always use the multi-weight oils for the reasons Dan posted. Once oil consumption stabilizes after overhaul / break-in, any of the AD oils that are recommended by the engine people will be good. Sticking with what the engine manufacturer or builder recommends and you will not go wrong. Just like primers, you will get as many different opinions on oil as there are oils. There is no reason to switch back and forth between straight and multigrade. Lycoming says 20W50 or 15W50 are "all temperature". 15W-50's higher VI simply means it is less thick when cold and less thin when hot. 100W offers no additional protection in the summer. ![]() So, yes, the multigrade is much superior in the cold, with no compromise when hot. Among other little details, it means the oil filter bypass is less likely to be opening.Īt 100C (212F), they are 20.2 and 19.6.no practical difference. Ever looked up the kinematic viscosity values for Aeroshell W100 Plus and Aeroshell 15W-50?Īt 40C (104F), they are 200 and 140, the multigrade being less of course.
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