The ASTM standard D 2502-92 (reapproved 2004) provides a means of calculating the mean molecular weight (relative molecular mass) of petroleum oils from kinematic viscosity measurements. It is applicable to samples with molecular weights in the range from 250 to 700 g/mol and is intended for use with average petroleum fractions. Nevertheless, this estimation was carried out by using a viscosity-molecular weight chart that involves evident interpolation errors. In this work, we have used both of Hirschler's equations as numerical methods in order to carry out an analytical estimation of the mean molecular weight of petroleum oils from kinematic viscosity measurements. The data provided by the named Hirschler-Maroto equation are in excellent agreement with the ASTM chart for a viscosity index range that covers the majority of commercial oils. Finally, by using the Hirschler-Maroto equation we have designed a PC program in order to make the evaluation of molecular weight of petroleum oils easier for engineers and professionals. The use of this program, which can be downloaded free from the Internet, only requires the insertion of kinematic viscosity data, which remove any graphical or interpolation errors. Nevertheless, the Hirschler-Maroto equation and the PC program must be cautiously used in order to avoid important errors when applied to the higher molecular weight constituents of petroleum.