Technical Abstract: The film thickness of seven seed oils and two petroleum-based oils of varying chemical structures, was investigated by the method of optical interferometry under pure rolling conditions, and various combinations of entrainment speed (u), load, and temperature. The measured film thickness (h measured) were then compared to that calculated (h calculated) using the Hamrock-Dowson (H-D) equation. The result showed that the H-D equation adequately and without exception predicted h measured in the high entrainment speed region (u > 0.1 m/s), but not in the low entrainment speed region (u < 0.1 m/s). In order to understand these deviations further, the chemical structural variabilities of the oils related to polarity (functional groups), unsaturation (double bonds), and molecular weight (chain length) were determined using quantitative empirical parameters. The effect of variability in the viscosity and chemical structure parameters of the oils on film thickness in the low entrainment speed region was then examined. The result of this examination indicated that, in the low entrainment speed region, the film thickness of seed oils: (a) rarely correlates with viscosity as proposed by the H-D equation, (b) increased with decreasing polarity of the oil, and (c) increases with decreasing degree of unsaturation of the oils.