Efficiency enhancement of various propulsion systems is in the focus of technical development, with the aim of reducing both operating costs and emissions. When considering internal combustion engines, only a fraction of the supplied fuel energy can be converted to mechanical work, whereas the remaining energy is mostly being released to the environment as waste heat. Organic Rankine Cycles (ORC) enable the use of this very waste heat for increasing overall efficiency of the propulsion system. To use the full potential of the waste heat recovery system, the impacts of the single influencing factors within the ORC have to be understood, such as the working medium, process management when using a regenerator, the number and arrangement of the single heat sources, the lower temperature level of the cycle (condensation temperature) or the share of circulating lubricant within the working cycle. This paper wants to assess the thermodynamic influence of the lubricant in an ORC in order to show the maximum potential of efficiency.