In this study, the lubricant viscosity from a diesel engine operating on various percentages of coconut oil blended fuel is investigated to determine the suitable mixture appropriate for a diesel engine. The coconut oil was blended with conventional diesel fuel at an amount varying from ten to fifty percent. The lubricant used in this study is the conventional commercially available lubricant (SAE 40). After proper blending of diesel with coconut oil, the blended fuels are loaded into the fuel tank. The engine is operated at half throttle setting running at 1600 rpm for 100 hours. At every ten hour interval, one lubricant sample was collected and analyzed using a viscometer at 100 °C and 40 °C. The results show that the viscosity for all the blended fuel remain about constant throughout the 100 hours test. However, fifty percent coconut oil blended fuel at 40 °C shows slightly lower viscosity level in comparison to other percentages of blended fuel system. It is expected this is due to high fuel dilution in lube oil. As a result, ten to thirty percent coconut oil blended fuels seems suitable for diesel engine because it's able to prolong lubricant life span by maintaining its viscosity.