When biodiesel finds its way into engine oil, it causes dilution of the oil, increasing viscosity and causing the lubricating oil to oxidize, leading to premature formation of sludge and deposits in the crankcase, resulting in a short period of the oil drain interval. This study was carried out to use adsorption as a means of mitigating this negative impact of the use of biodiesel on crankcase oil by selectively removing oligomers formed in the oil as a result of degradation of biodiesel. It was also to determine a suitable adsorbent for the use of this purpose. Neat base oil mixed with rapeseed oil methyl ester (RME) biodiesel (20% RME and 80% neat base oil by volume) were thermo-oxidatively aged at 170 degrees C for 80 h. Also, unaged longlife engine oil was used in this study to observe the impact of the adsorbents on additives. The adsorbents used in this study were 20% dimethyldichlorosilane (DMDCS) chemically deactivated silica gel, 20% DMDCS-deactivated alumina, and 25% water deactivated silica gel. A total of 20 g of the selected adsorbents was used to create an adsorption bed in a chromatographic column for the separation process. The total acid number and changes in molecular masses before and after separation were determined. The adsorption data of oligomers onto adsorbents were attained and described by Langmuir and Freundlich adsorption isotherms. The thermodynamic parameter, Delta G degrees, was calculated, which indicated that the adsorption was spontaneous in nature. The adsorption also followed both surface adsorption and intraparticle diffusion mechanisms. The total acid number of the aged base oil and RME mixture without separation, 10.46 mg of KOH/g, was reduced to 1.03, 1.49, and 7.37 mg of KOH/g by 20% DMDCS-alumina, 25% water-deactivated silica, and 20% DMDCS-silica, respectively. The 20% DMDCS-alumina adsorbed 57% of oligomers, 25% water-deactivated silica adsorbed 48% of oligomers, and 20% DMDCS-silica adsorbed 37% of oligomers. The results of this study have shown that adsorption is a potential useful tool for oligomer removal from aging engine oil.