Universal base oils that remain pourable over wide temperature ranges would have important advantages for lubrication applications. The model system used in this project was a poly(alpha-olefin) synthetic base oil modified with polydimethylsiloxane (PDMS) to lower the pour-point temperature. Although the blend was miscible at room temperature, phase separation occurred at temperatures lower than 258 K. Partition coefficients of such nonideal oligomer mixtures can (1) help define operating temperature ranges and (2) provide a basis for designing molecular weight distributions of each lubricant that control or prevent phase separation. The poly(alpha-olefin) base oil family is branched oligomers with two to five n-mers at levels greater than 1 wt %, whereas PDMS additives are linear oligomers having between 10 and 50 sequential n-mers at levels greater than 0.5 wt %. In this study, Fourier transform infrared measurements of the poly(alpha-olefin) and PDMS compositions in each phase provided an overall material balance. Poly(alpha-olefin) oligomers were detected with size exclusion chromatography with a differential refractive-index detector, and PDMS oligomers were detected with matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry. The best sets of measurements for the individual oligomers in each phase were selected by minimization of the overall material balance errors. For both oligomers, components with high molecular weights were preferentially excluded from the phase rich in the other polymer and were relatively independent of temperature. The partition coefficients of poly(alpha-olefin) components increased with increasing oligomer length, whereas the partition coefficients of the PDMS components decreased with increasing oligomer length. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122:2915-2925, 2011