The viscosity index (VI) is a useful tool for lubricant users and refiners, since it is a measure of the effect of temperature changes on the viscosity of the oil. However, it was found that the viscosity index does not correlate with the flow activation energy E(a), which is the theoretically defined dependence of the viscosity on temperature. In this way, two oils may have the same flow activation energy but a viscosity index varying by up to 120. We therefore believe that the VI does not always give a proper representation of the effect of temperature on the kinematic viscosity. (13)C NMR spectroscopy was used to identify the molecules with a high VI. Twenty different oil samples produced from eight different vacuum gas oils with viscosity indices ranging from -104 to 146 were analyzed and key parameters identified for high VI molecules: long alkyl chains, methyl branching in the centre of the molecule, low content of aromatic compounds, no ethyl branching and no tertiary carbons. A correlation based on four selected peaks was developed, giving a very good prediction of the viscosity index. (C) 2009 Elsevier Ltd. All rights reserved.