Everything You Need To Know About Multi-Viscosity Oils

Before we get into whether you should be using a multi-grade or single grade motor oil, some definitions are in order. Viscosity has been defined as that property of a liquid which causes it to show a resistance to flow. The resistance is important because motor oil must adhere very tenaciously to the moving parts of the engine. As temperatures decrease, lubricating oils become thicker, heavier and show greater resistance to flow — in short, they become more viscous. Conversely, the same oils become thinner, lighter and show less resistance to flow in other words, they become less viscous as temperatures increase. The amount of viscosity can be critical to engine maintenance and performance especially in severe temperatures. It is the viscosity of the motor oil which determines the temperature below which the engine cannot be started in cold weather. It also is the determining factor in the thickness of the oil film on the engine. If the oil film is so thin that it cannot keep moving parts protected, wear results. The grades of viscosity in crankcase oils have been classified by the Society of Automotive Engineers (S.A.E.). The system of classifications developed from the use of the Saybolt viscometer which measures the flow of the crankcase oils in seconds through a standard orifice at atmospheric pressure and at a predetermined temperature. Under these classifications, 5W (for winter), 10W and 20W are determined by the oils’ viscosity at 0° Fahrenheit, while grades 20,30, 40 and 50 are determined by its viscosity at 210° Fahrenheit. While all lubricating oils become more viscous as temperatures fall, and less viscous when temperatures rise, they do not change in viscosity to the same degree. The rate of change in viscosity with the change in temperature is known as the Viscosity Index (VI). For example, two oils of the same SAE grade which have roughly the same viscosity at 210° Fahrenheit may have radically different viscosities at higher and/or lower temperatures than 210° F. The oil showing the least change is the most desirable because it results in lower consumption, less engine wear and makes possible the starting of an engine at lower temperatures. Some crankcase oils with high VIs, however, are the result of additive agents, which lose their effectiveness in use. The oil to look for, therefore, is one which in its crude state and after its refining measures high on the VI scale. Pennsylvania Grade Crude, from which Kendall Motor Oil is refined, has the highest VI of all oils produced and marketed. In its crude state, its VI is 95-100 and after refining its index averages 100-115. (Some Kendall oils have been refined to a VI as high as 140 without the addition of improving additives). In comparison with Pennsylvania Grade Crude, Mid-Continent shows a VI of 65-75 before refining and 80100 after refining; East Texas’s comparative VIs are 50-60 and 80-95 and California’s are 0-35 and 40-65. Kendall now is blending base oils with natural VIs on the 130 range. As a result its crankcase oils are superior in load carrying ability, anti-wear characteristics and sealing. At the same time, these oils require a much smaller quantity of VI improver. The VI improver additives, like the base oils, vary widely in stability. The more expensive these polymer additives are, the more stability they will provide for the required viscosity of the motor oil. Because Kendall’s crude is so high on the VI scale, it can economically use the more expensive, more stable types of improvers. Now to the question of whether to use multi-viscosity or single graded oil. The advantages of multi-viscosity oils become less significant in direct relation to the increase in ambient or start-up temperatures. When start-up temperatures exceed 70° Fahrenheit, it is questionable whether a multi-viscosity oil has any advantages over single-grade lubricants of equal quality. High performance engines when driven as high output engines require special lubricants such as GT-1. The exception is cold weather operation when engine start-up becomes difficult with an SAE 20-20W. During the cold weather period a lubricant such as Superb 10W-30 or 10W-40 should be used. While the engine is lubricated with a multi-viscosity oil, it should not be asked to perform at its highest level. A single graded SAE 10W GT-1 may be used to facilitate cold starting but care must be exercised not to demand excessive performance. Superior over-all protection would be obtained from Superb 10W-30 or 10W 40 as compared to GT-1 SAE 10W. Generally speaking, we should recommend single viscosity oils for high performance except when low temperature start-ability is a problem. Author: Thomas T. Ordiway
Kendall Refining Company


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