What is the density of transformer oil?

Density of Transformer Oil at 29.5oC is 0.89g/cm3.

The exact density of transformer oil depends upon the manufacturer but, expressed in SI units, is typically a little less than 900 kg/m3 at 20oC. The J&P Transformer Book states that 'a limit of 895 kg/m3 at 20oC ensures that the temperature (of the oil) must fall to -20oC before the density of the oil would exceed that of ice' -thus ensuring that if ice forms, then it would remain at the bottom of the tank.