In this study, performance of NPNA (non-phosphorous and non-ash) engine oil was investigated experimentally. A field test was conducted with 2 identical SI engine powered commercial taxi vehicles. A conventional, P-containing SAE 5W-40 synthetic engine oil and NPNA SAE 5W-40 synthetic engine oil were used for evaluation. Vehicles operated during a single oil drain period which was predefined as trip length of 15 000 km by the manufacturer and the engine oil samples were taken every 2500 km. 3-way catalysts were dismantled from the vehicles and analysed individually at the end of the test. Elements on the wash-coat surface of catalyst were investigated by using electron probe microanalyser (EPMA). According to elemental analysis of the catalyst surfaces, P-containing engine oil revealed higher levels of Ca, Zn and P than NPNA engine oil. Results of used oil analyses showed that the total acid and base number (TAN-TBN) data of NPNA oil were not satisfying the conventional standard whilst the wear metals, viscosity, and water content were in acceptable limits at the end of the test. This shows that NPNA oil can be used as substitute for conventional engine oils and would not terminate the level at what the driver should change oil.