Technical Data
Viscosity Index (VI) is a measure of how oil viscosity changes with variations in temperature. The lower the VI, the greater the change of viscosity of the oil with temperature, and vice versa.
The VI scale was set up in the 1920's by the Society of Automotive Engineers (SAE). The temperatures used for reference are 100 and 210F (38 and 99C). The original scale only stretched between VI=0 (lowest VI oil, naphthenic) and VI=100 (the then best oil paraffinic) but since the conception of the scale better oils have been produced, leading to VIs greater than 100.
The viscosity of oil decreases as temperature increases, but each will only be at the 'optimal' viscosity for a given application over a certain temperature range. The best oils with the highest VI will remain stable and not vary much in viscosity over a wider temperature range, increasing the window within which optimal lubrication can be achieved. Multigrades will always have higher VI's than monogrades, which is one reason why monogrades typically need to be changed between winter and summer.
ZDDP (full name Zinc dialkyldithiophosphate) is a compound developed in the 1940s consisting of zinc bound to the anion of dithiophosphoric acid. They are soluble in nonpolar solvents, and the longer chain derivatives easily dissolve in mineral and synthetic lubricating oils.
The main use of ZDDP is as an anti-wear additive to lubricants, but over time their concentrations have been reduced to avoid damaging catalytic converters on modern vehicles. Crankcase oils with reduced ZDDP cause damage to, or failure of, moving parts lacking 'full film' lubrication in classic vehicles such as flat tappet camshafts and followers, which undergo very high boundary layer pressures and/or shear forces at their contact faces; and in other areas such as piston rings and gudgeon pins. The same ZDDP compounds serve also as corrosion inhibitors and antioxidants.
ZDDP concentrations deplete with use, and are one factor in determining the change frequency of an engine oil.
Base Oil Group describes the nature of the raw oil used in the blending of a lubricant.
Chemical substances (additives) are added to the base oil in order to produce a lubricant with further improved anti-friction, anti-corrosion and cleaning properties.