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Engine Oils Background & Properties castrol.com/GPS 800 255 4417 May 2025 GPS Industry Specifications High Temperature/High Shear (HT/HS) An oil's resistance to permanent viscosity loss due to shear is called shear stability. Forces inside an engine — such as high temperature, heavy loads and outside contaminants — can cause an oil to shear down, or thin. Oil thinning can cause increased oil consumption and reduced oil film strength. Once the oil film strength is compromised, engine components are exposed to wear and premature failure is possible. Increased oil consumption can also elevate maintenance costs. Oils that are shear stable maintain their oil film thickness, protecting critical moving parts like rings, bearings, and other engine components while extending engine life. Temporary viscosity loss in high-speed, heavy-load applications can result in metal-to-metal contact, which causes premature engine wear. Oils that offer excellent high temperature/high shear protection give users an increased margin of protection against viscosity loss. Viscosity Index (VI) Viscosity index is a measure of an oil's change in viscosity based on changes in operating temperatures. Oils with a high viscosity index (over 100) experience relatively little change in viscosity as a result of temperature changes. High VI oils do not experience thinning at high temperatures or thickenening at low temperatures. Viscosity index is a performance benefit that is a result of high-quality base oils and premium additives. Base Oil Building a Lubricant from the Base Up Base oil comprises 80 to 98 percent of finished lubricants used in engines, transmissions, hydraulics, and gear housings. Base oil contributes to a number of properties in a finished lubricant; these properties can be further enhanced with additives. A lubricant manufacturer's goal is to accentuate the base oil's positive properties while decreasing or removing any unwanted characteristics. Some of the most important properties influenced by a base oil are listed and defined below: Viscosity and Viscosity Index (VI) Viscosity is a measure of a fluid's resistance to flow; viscosity index (VI) is the relationship of a fluid's viscosity to temperature. High VI fluids experience less change in viscosity relative to temperature than low VI fluids do. Base oils have natural tendencies toward high, medium or low VIs, but VI can be enhanced through refining techniques and with additives in the blending process. Volatility Volatility refers to how readily a base oil and resulting finished lubricant will evaporate as temperatures rise. Volatility is especially important in engine oils where compartment temperatures are extremely high; high volatility can result in increased oil consumption. Pour Point Pour point is the temperature at which an oil "gels." Pour point directly influences a finished lubricant's ability to flow to critical components during low ambient temperatures. In a base oil, the amount of wax in the fluid can make pour points higher, but refining techniques can remove the wax so that the pour point is lowered. It is recommended that a finished lubricant's pour point be approximately 15 degrees below the expected ambient temperature. A18 ecastrof