This research explores the utilization of waste cooking oil (WCO) biodiesel as a substitute fuel in a diesel engine. The study investigated the use of different biodiesel-diesel blends having different Lower Calorific Values (LCV). The Engine was tested at different Engine Load (EL) to assess its Brake Specific Fuel Consumption (BSFC) and Brake Thermal Efficiency (BTE). To find an optimal balance between BTE and BSFC, the Response Surface Methodology (RSM) was used for modeling and establishing the link between the control factors (EL and LCV) and the response variables (BTE and BSFC). The results demonstrate that both higher EL and higher LCV make BTE better and BSFC lower. The best EL was 94.57%, the best LCV was 41558.29 kJ/kg, the best BTE was 30.10%, and the best BSFC was 0.191 kg/kW-hr. The desirability score was 0.989. The study backs up the idea that WCO biodiesel is an effective alternative fuel that helps achieve SDG 7 and net-zero targets. RSM worked efficiently find the best engine settings for the best performance, therefore this approach may be used with a lot of other types of engines and alternate fuels.