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.  

There are five groups:
  • Group I base oils are classified as less than 90 percent saturates, greater than 0.03 percent sulphur, and with a viscosity-index range of 80 to 120. Group I base oils are solvent-refined, which is a simpler refining process. 
  • Group II base oils are more than 90 percent saturates, less than 0.03 percent sulphur, and with a viscosity index of 80 to 120. They are often manufactured by hydrocracking, which is a more complex process than that used for Group I base oils. Since all the hydrocarbon molecules of these oils are saturated, Group II base oils have better antioxidation properties. They also have a clearer colour than Group I base oils. 
  • Group III base oils are greater than 90 percent saturates, less than 0.03 percent sulphur, and have a viscosity index above 120. These oils are refined even more than Group II base oils in order to achieve a purer base oil. Although made from crude oil, Group III base oils are sometimes described as synthesised hydrocarbons.
  • Group IV base oils are polyalphaolefins (PAOs). These synthetic base oils are made through a process called synthesising. They have a much broader temperature range and are suitable for use in extreme temperature conditions.
  • Group V base oils are classified as all other base oils, including silicone, phosphate ester, polyalkylene glycol (PAG), biolubes etc. These base oils are at times mixed with other base stocks to enhance an oil’s properties. Esters are common Group V base oils, which can take more abuse at higher temperatures and will provide superior detergency compared to a Group IV base oil.

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.