
58 Refining Used Lubricating Oils
2.4.2 viscOsity index
The viscosity index is strictly an empirical number and indicates the effect of change
in temperature on viscosity. A high viscosity index indicates a small change in vis-
cosity with temperature, which also means better protection of an engine that oper-
ates under vast temperature variations. Viscosity index improvers are among the
common additives that improve the efciency of the oil. However, engine oil with
a high addition level of viscosity index improvers tends to degrade more rapidly. A
high viscosity index is due to the absence of aromatic and volatile compounds and is
also an ind ...