For many
liquids, specific gravity is used
which is ratio of the mass of
a given volume to the mass of
an equal volume of water. Therefore,
specific gravity is dimensionless.
The specific gravity of mineral
oils also varies from 0.86 to
0.98 since the specific gravity
of water is 1 at 15.6°C. Specific
gravity decreases with increased
temperature and decreases slightly
as viscosity decreases for similar
compositions. Reference 5 (pp.
482 484) gives the specific gravity
of 81 mineral oils at 15.6°C.


Most lubricant
supplier's typical data bulletins
give A.P.I. (American Petroleum
Institute) Gravity in degrees for
lubricating oils instead of specific
gravity. A.P.I. gravity is an expression
of density measured with a hydrometer.
A.P.I. gravity has an inverse relationship
with specific gravity: 



Many mineral
oil lubricants have an A.P.I. gravity
value of around 27 degrees. Reference
8 gives the equation for converting
A.P.I. gravity to specific gravity. 

Density, specific
gravity, and A.P.I. gravity are
measured by ASTM D1298, using a
calibrated, glass hydrometer and
a glass cylinder. The cylinder is
partially filled with the sample
oil and the hydrometer is set into
the oil and allowed to stabilize.
A reading of the gravity is taken
from the markings on the stem of
the hydrometer at the surface of
the oil. The temperature of the
oil is measured and the final result
is converted to 15.6 °C (60
°F) and reported as A. P. I.
gravity at 60 °F. 

Two other oil
properties related to density are
thermal expansion and bulk modulus
or compressibility. 