is an indicator of the approximate hydrocarbon composition of a petroleum oil. As described in test method ASTM-D-2501, VGC is calculated from one of the following equations, depending on the temperature at which viscosity is determined (VGC at 37.8°C [100°F] is the preferred equation):

VGC @ 37.8°C (100°F) =
                                  (10G —1.0752 log (V-38))/ 10-log (V-38)

where:
G = specific gravity @ 15.6°/15.6°C (60/60°F);
V = Saybolt Universal viscosity @ 37.8°C (100°F).

or
VGC @ 98.9°C (210°F) =
 (G—0.1244 log (V1-31))/( 0.9255 - 0.0979 log (V1-31)) - 0.0839

where:
G = specific gravity at 15.6°/15.6°C (60/60°F);
V₁ = Saybolt Universal viscosity at 98.9° (210°F).

Values of VGC near 0.800 indicate an oil of paraffinic character; values close to 1.00 indicate a preponderance of aromatic structures. Like other indicators of hydrocarbon composition (as opposed to a specific laboratory analysis), VGC should not be indiscriminately applied to residual oils (see bottoms), asphaltic materials, or samples containing appreciable quantities of non-hydrocarbons

Also see Saybolt Universal Viscosity, specific gravity.