This document presents a methodology to select the optimum blend of waste plastic oil (WPO) and diesel using a combinatorial mathematics based approach (CMBA). Experimental data on performance parameters like brake power, specific fuel consumption, mechanical efficiency, brake thermal efficiency and indicated thermal efficiency is collected for blends of 10%, 20% and 30% WPO at different engine loads. CMBA involves normalizing the data, assigning relative importance weights to parameters, forming an alternative selection matrix, and calculating the permanent function value to determine the optimum blend. The analysis found that the 20% WPO blend at a 10kg load has the highest permanent function value and forms the optimum blend out of all test fuels.