Viscosity Index (VI) Test
The Viscosity Index (VI) Test (ASTM D2270) is based on the kinematic viscosity of the fluid at 104 ° F (40 ° C) and 212 ° F (100 ° C). Fluids whose viscosities do not change much between these two temperatures will have higher viscosity indices than those whose viscosity changes are greater. Viscosity index numbers above 95 are considered high.
AMSOIL Advantages RE: High Viscosity Index
High Viscosity Index AMSOIL lubricants are formulated to have naturally high viscosity indices, so the need for viscosity index improvers is reduced. The VI improvers used in AMSOIL lubricants are temperature specific, meaning they are activated only when certain temperature requirements are met. In most cases, VI improvers help maintain thickness at higher temperatures while having minimal effect at low temperatures. By using viscosity improvers with a high shear-stability index, AMSOIL is able to achieve optimal cold-weather performance with virtually no loss to shear-stability performance. AMSOIL lubricants resist thinning at high temperatures (high VI) and can suppress the generation of additional friction and heat generated by components in contact due to a thinning lubricant. AMSOIL Synthetic Heavy-Duty Diesel Oil (ACD) and Synthetic Small Engine Oil (ASE) meet multi-grade viscosity requirements without the use of viscosity modifiers because their synthetic base oils have naturally high viscosity indices and are wax-free. These oils meet both the low-temperature requirements of SAE 10W and the high-temperature requirements of SAE 30, allowing the oil to perform adequately at both hot and cold temperature extremes.
Thermal Stability – AMSOIL synthetic base oils have better thermal stability than mineral oils. Thermal Stability permits the oils to be used longer, even as speeds and temperatures increase. It also allows oils to retain their viscosities at low temperatures. Lower-viscosity oil provides better cold-weather operation, allowing the oil to be quickly circulated at cold-temperature startups and supply engine components with the proper lubrication to keep them protected.
OilBuddy’s Take
In other words, a higher Viscosity Index (VI) number provides the benefits of maintaining viscosity at higher temperatures which results in less wear and oil consumption. This also benefits the oil viscosity at lower temperatures with improved starting and better fuel efficiency.