Abstract: Well understanding the impact of molecule compositions on the viscosity index of lube base oil is very important for feedstocks selection and operation adjustment. With the combination of traditional and high resolution mass spectroscopy, nuclear magnetic resonance characterization methods, the relation of viscosity index and molecule compositions of four samples (feed A1 and A2, base oil product B1 and B2) was studied. The results showed that B1 and B2 have lower content of paraffin, lower iP/nP, but higher content of naphthene, resulting in lower viscosity index than A1 and A2. The content of alkanes in A1 and B1 is lower and the content of naphthene with different ring numbers is higher than in A2, B2, respectively, A1and B1 still have higher viscosity index because they have more carbon number in the same distillate range. The properties and constituents of narrow fractions indicated that the viscosity index of narrow fraction increases with the end point and the fraction with higher viscosity index has higher contents of paraffins and one ring naphthenes.

Key words: base oil, viscosity index, paraffin, naphthene