Since the early days of the petroleum industry, some physical properties of crude oil were used to define characterization factors for classification of crude oil with respect to hydrocarbon types  as shown in Equation 8.
where: Tb = volume, or mean average normal boiling point in R (degree Rankine) and SG = specific gravity at 15.6°C (60°F). To calculate KUOP or KW, volume average boiling point (VABP) or mean average boiling point is used, respectively. Depending on the value of the Watson characterization factor, crude oils are classified as paraffinic (Kw = 11-12.9), naphthenic (Kw =10-11), or aromatic (Kw <10).
Another parameter defined in the early years of petroleum characterization is the viscosity gravity constant (VGC). This parameter depends on viscosity expressed in Saybolt Universal Seconds (SUS) and specific gravity. According to a standard method (ASTM D2501), VGC can be calculated at a reference temperature of 100°F as follows in Equation 9:
where V(100°F) is the viscosity in SUS and SG is the specific gravity at 15.6°C (60°F). VGC varies between 0.74 to 0.75 for paraffinic, 0.89 and 0,94 for naphthenic, and 0.95 and 1.13 for aromatic hydrocarbons.
The U.S. Bureau of Mines Correlation Index (BMCI) or (CI) is useful for characterization of crude oil fractions. CI is defined in terms of Mean Average Boiling Point (Tb) and specific gravity (SG) at 60°F as shown in Equation 10:
According to this CI scale, all n-paraffins have a CI value of 0, while cyclohexane (the simplest naphthene), has a CI value of 50, and benzene has a CI value of 100. Using the CI values, crude oils can be classified as follows:
 K. M. Watson, E. F. Nelson , George B. Murphy, “Characterization of Petroleum Fractions,” Ind. Eng. Chem., 1935, 27 (12), pp 1460–1464