In recent times, many specially engineered diesel engines built in the 1980s are still used in different areas, for which the suggested engine oils are no longer available. These engines are operated reliably in military combat vehicles, electricity generators, and tracked machinery, thus it is essential to examine what engine oils are suitable for their operating conditions and may be used as their engine oil. The aim of our research is to conduct the tribological examination of an oil sample taken from a V-6M type naturally aspirated Diesel engine and the oil prescribed by the manufacturer. After evaluating the results, we determine which type of oil is applicable in the engine to achieve safe operation and a long lifetime.