With the trend towards smaller and high power-density electrical machines, the thermal design of the machine is becoming increasingly important. An innovative cooling technology is the oil spray cooling, where the critical components of the machine (end windings) are directly wetted with the coolant using pressure atomizers. The thermal design process of spray cooled electrical machines requires reliable heat transfer correlations. Therefore, in this contribution the single-phase convective heat transfer of spray cooled plain surfaces was investigated to establish a foundation for future investigations. Multiple operating parameters such as the temperature of the coolant, the nozzle, the pressure drop across the nozzle and the distance between nozzle and surface were varied. As for the coolant, aqueous glycerol solutions and a transmission oil of type ATF VI were used. Based on the measurement data, a heat transfer correlation was derived which applies to highly viscous liquids with Prandtl numbers ranging from 80 to 340. The heat transfer correlation describes the data with a mean absolute percentage error of 15%.