Tests of imbibition rates in Berea sandstone cores have been used to examine the wettability effects of displacement of crude oils with a number of synthetic base oils. In all cases examined to date, the products that flocculated asphaltenes from crude oils also caused water-wet sandstone cores to become markedly less water-wet or even oil-wet. Relatively minor effects on wetting were observed with base oils that did not destabilize asphaltenes. A simple diagnostic test of asphaltene stability can be used to identify specific crude oil/base oil pairs that precipitate asphaltenes.