Supercritical power cycles offer a unique opportunity to utilize low temperature heat sources to reduce waste heat and offer new opportunities for renewable energy sources. Although it is established that supercritical power cycles have high efficiencies at low temperatures, the real gas effects which lead to this have not been comprehensively studied. Here, we study the impact of real thermophysical properties on the relation between efficiency and turbine inlet temperature for Brayton cycles. We introduce an infinitesimal cycle approach to demonstrate that the ratio of specific heat at constant pressure across an isentropic expansion process leads to both rapid increases in efficiency for power cycles with compressor inlet conditions in close proximity to the critical point as well as decreasing efficiency for all Brayton cycles above a given optimal turbine inlet temperature, the latter deviating from current literature. Additionally, we provide insight into the mechanisms, driven by the real gas effects, which result in the interaction between efficiency and turbine inlet temperature. Finally, we provide temperature maps providing the turbine inlet temperature at which highest efficiency is achieved for the carbon dioxide from supercritical to sub-critical conditions for varying isentropic efficiencies.