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 D-1298, 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.