The specific gravity is the ratio of the density of oil to the density of water. Since the density of water is 1g/ml, the density of a substance is numerically the same as the specific gravity.
However, specific gravity is a dimensionless number, while density has units mass/volume.
So, the density of oil is .901 g/ml, but the specific gravity is just .901
In Imperial units, the density of the oil is 56.2 lb/ft^3, but the specific gravity is still .901
