Density is
the mass of a unit volume of a substance.
Oil density is used to determine
the mass of a given volume, or the
volume of a given mass. Density
is used in lubrication to identify
an oil, or oil fractions, and in
the measurement of kinematic viscosity
(absolute viscosity divided by density).
Also, density is in the equations
for the calculation of temperature
rise in an oil film, and the equation
for Reynolds Number (which determines
if flow of an oil film is laminar
(smooth layers) or turbulent (tumbling)).