For large-scale implementation of biodiesel in transportation engines, its effect on lubricating oil degradation needs to be experimentally investigated. Due to differences in chemical composition of biodiesel and mineral diesel, the comparative effects on lubricating oil degradation and residual useful life for long-term application will be different. In this study, effect of 20% Karanja biodiesel blend on lubricating oil tribology was studied vis-a-vis mineral diesel in a 200 h long endurance test. Higher increase in density, carbon residue and ash content was observed for biodiesel blend fuelled engine's lubricating oil in comparison to diesel. Higher amount of resinous polymerized material in the lubricating oil of biodiesel fuelled engine indicated possibility of higher oxidation and polymerization of base-stock of lubricating oil of the biodiesel fuelled engine. After 100 h, higher increase in concentration of wear trace metals such as iron, aluminum, copper, chromium and magnesium in the lubricating oil of biodiesel fuelled engine in comparison to mineral diesel indicated significant deterioration of lubricating oil. (C) 2014 Elsevier Ltd. All rights reserved.