The operational life and reliability of internal combustion (I.C.) engines are limited by the breakdown of the engine components due to wear under boundary lubricated conditions. It is very advantageous to know the condition of an engine and its components without disassembling the engine for examination. This paper employs the chemical and physical analysis of used synthetic crankcase oil to predict the condition of the lubricant and engine wear components during continuous operation. In this research, it is concluded that the iron content of the used oil can be used to define the oil drain interval to optimize oil use in the engine and minimize the wear process. Using an engine dynamometer, the optimum oil drain interval for a fully synthetic 15W50 was found to be 11,000 km.