Diesel engines equipped with Diesel Particulate Filters (DPF) suffer from the carry-over of fuel into the engine oil that causes oil dilution. Oil dilution can lead to a decreased oil performance and oil durability. Diesel fuel slowly evaporates out of the engine oil, and does not remain in the oil pan. Compared to diesel fuel, biodiesel remains nearly completely in the engine oil, and leads to a constant engine oil dilution. Furthermore, biodiesel is able to react with the engine oil that can lead to formation of oil sludge, which can result in severe engine damage. The common strategy, which solves those problems, is the reduction of the oil drain interval that leads to increased engine oil consumption. We introduce a method to remove biodiesel from the engine oil by use of an entrainer in order to extend the oil drain interval. Fourteen blends of biodiesel, engine oil and entrainer were distilled, and the distillate was analyzed by gas chromatography and mass spectroscopy for quantitative amount of biodiesel. The influence of polar hydroxyl groups, the dipole moment and the chemical structure of fourteen entrainers to the drag-out of biodiesel were compared. In addition, we analyzed the activity coefficient of the entrainers used with biodiesel, and its influence of the amount of removed biodiesel as well as we determined the present interactions between entrainer and biodiesel. Furthermore, the measured activity coefficients were simulated by a quantum chemical method to compare the experimental results for consistence and analyze the possibility to predict the interaction of entrainer and biodiesel. The results show the drag-out of biodiesel from the engine oil at temperatures of 160 degrees C and 180 degrees C by use of polyols and acids. Mono-and bivalent alcohols yield no drag-out. The number of the hydroxyl groups or the dipole moment of the entrainer does not correlate with removed biodiesel. Acids showed the highest ability for the drag-out of biodiesel (2.3% (m/m) in total for formic acid) and showed the strongest interaction with biodiesel (for formic acid). According to the results, this strong interaction is mainly formed by hydrogen bonds. Furthermore, biodiesel turns out to be a good hydrogen bond acceptor and can be removed by substances that are hydrogen bond donators (e.g. acids, polyols). (C) 2013 Elsevier Ltd. All rights reserved.