The main challenge in today's modern engines is to design the parts, which should withstand higher temperatures. To achieve this, selection of materials and process tolerances are very important factors. The product identified in this study is a conventional oil pump, which is an engine auxiliary component. The function of the oil pump is to supply oil to different parts of the engine to lubricate and reduce the overall engine friction. The different speed and load conditions for which the engine is subjected, pose a challenge to the oil pump, to supply the necessary quantity of oil at the required pressure and temperature. Normally, the oil pump is subjected to a temperature of 120°C at higher speeds. However, the peak oil temperature in modern diesel engines can be as high as 140°C to 150°C for a short period of time.For this study, two engine grade oils were selected. Numerical analysis was performed to predict the oil flow rate for these oil grades. In addition to this, numerical analysis was performed for optimization of oil pump clearances. Numerical results were validated with experiments with an accuracy of approximately 5%. Proto samples were made with this optimized clearance and tested along with existing clearances under steady state conditions. Engine test was also carried out to measure the oil pressure. The results show that by lowering the oil viscosity (SAE 0W20), the oil flow rate and pressure were reduced. By reducing the clearances in the rotor, the oil flow rate was matched with higher viscosity oil (SAE 5W30). The oil pressure was also increased by approximately 10% as compared with higher viscosity oil, which is favourable in reducing the priming time at cold start conditions. The power consumed at maximum power point was reduced by approximately 5%.