The diesel oil type S-3 specified for diesel engine has limited the suitability for diesel trucks for 8000 km, but didn't clarify its suitability if used in tractor engines.It is known that the work style of farm tractor differs from that of other vehicles where tractors are used for all the activities in sever conditions and under the complete usage of the available power and capability, so there is no sign or indication of the usage period of this oil in tractor's engine. The oil has been used on Cirta C6806 tractors. The manual book of the tractor's engine, Deutz recommends changing the oil every 100 hrs. Therefore the main goal of this research is to give the recommended working hours for S-3 diesel oil when used in farm tractor engines. Five tractors have been used in this research and the engine oil was changed for the interval period of 20 hrs starting from 20, then 40, 60, 80, 100 and 120 hours. At the end of each period the oil of the five tractors was mixed and a sample was taken for testing. This procedure was repeated three times in order to obtain accurate results with statistical analysis. The statistical analysis results showed that the S-3 diesel oil deteriorates after 40 working hours because of the great consumption of the oil after this period and therefore this research recommends that the usage period of S-3 oil should not exceed more than 40 working hours when it is used in tractor's engine.
