The fluctuation theory equation of state has been applied to the class of chemicals used for lubrication in extreme conditions ranging from the cold vacuum of outer space to the high pressure and high temperatures of deep oil wells. The predictions made by this equation agree very well with the experimental data considering the input data include only temperature-dependent density and isothermal compressibility at atmospheric pressure to predict the high-pressure and high-temperature densities. The densities of a total of 56 different oils were analyzed with this equation up to a temperature of 533 K and pressure of 1 GPa. The range of deviation for the measured samples is from 0.01% to 4.31%, and the physical basis for this deviation is discussed.