To make 5W-50 oil, a manufacturer starts with a light viscosity base stock and adds a relatively large volume of viscosity index improver up to 15 percent. Superior quality oils, such as AMSOIL synthetic motor oils, are formulated with shear stable viscosity index improvers that enhance the oil’s high and low temperature performance. Bargain basement oils tend to have low quality viscosity index improvers. Multi-grade oils are not equally subject to shear back or cold temperature thickening, their ability to resist these harsh environments depends on the quality of their viscosity index improver chemistry. High rates of shear occur in areas where the oil is forced through narrow passages in an engine such as between the piston rings and cylinder walls. When tangled polymer chains are subjected to a high rate of shear, they are prone to rupture, a phenomenon called “shear back.” Oil that has sheared back has a permanent loss of viscosity and no longer provides a thick oil film for protection in high speed, high load or high temperature operations. Subject to temporary attractions between non-adjacent molecules, tangled chains thicken the oil at low temperatures and impede the oil’s cold temperature pumpability.
Viscosity index improvers are subject to two important problems, both due to tangling of their long chain structure. For example some viscosity index improver side chains provide dispersancy while others do not. Motor oil base-stocks have 20 to 50 carbon atoms in their backbone chains and the major differences in viscosity index improvers come from their side groups that vary in chemistry or size. Viscosity index improvers may have as many as 2000 carbon atoms in their backbone chains. Viscosity index improvers known as polymers, chemicals typified by long repeated chains of molecules, emerging from the lab in the late 1940’s. But there’s a point at which viscosity index improvers become too much of a good thing – and several lubricant manufacturers may have breached that point with their newly offered synthetics. In both types of oil the viscosity index improver allows the oil to function over a wide temperature range, and its side groups may increase fuel economy, provide dispersancy, improve cold startability or reduce oil consumption. Due to their inherently high viscosity index, synthetic oils require less than petroleum oils do. High quality motor oils contain about four to six percent viscosity index improvers. This is why multi-grade oils exist.Ī small volume of a high quality viscosity index-improver upgrades the functionality of motor oil. However, low index number oil, say 90, would be quite different if it would become very fluid and thin and pour easily at high temperatures much like honey if heated to a higher than room temperature. Oil with a high index number, say 160, would look and behave similarly at these two temperatures. The index number indicates the degree of change in viscosity of an oil within a given temperature range, currently 40-100☌. The extent that they change is indicated by their viscosity index (VI). The key is to select a fluid that is not too light and also not too heavy.įluids thicken as their temperatures decrease and thin as their temperatures increase (like candle wax).
If you use too high, the fluid friction increases resulting in reduced energy efficiency, higher operating temperatures, and hard starting – particularly at cold temperatures. If you use too low a viscosity oil metal-to-metal contact occurs, poor sealing and increased oil consumption. The fact is, the use of high viscous fluids can be just as detrimental as using too light an oil. Knowing that a fluid’s viscosity is directly related to its ability to carry a load, one would think that the more viscous a fluid is, the better it can lubricate and protect. The fluid must be adequate enough to separate moving parts at operating temperatures of the equipment. The greater a fluid’s viscosity, the greater the loads that it can withstand. Viscosity is very important because it is directly related to a fluid’s load-carrying ability. Terms such as thick, heavy and high suggest the fluid demonstrates a strong resistance to flow in the example such as honey.
Other ways to refer to a fluid’s viscosity in more common generic terms are thin, light and low and suggest that the fluid flows readily such as water. The term used to describe this is Apparent Viscosity and it is expressed in units known as centipoise (cP). However, significantly more energy is required to stir honey with that same spoon. It takes little energy to stir water with a spoon. You can think of this as the energy required to move an object through the fluid. A fluid’s viscosity can also be indicated by measured resistance.
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