This paper deals with the study of the effect of fluid viscosity on the ability to generate and propagate self-excited oscillations in the inner space of a fluidic oscillator and at its outlet to the air environment. The generation and propagation of self-excited oscillations was monitored at several levels of inlet pressure values (5, 10, 15, 20 and 25 bar) and two flowing fluid consternations of the ethylene glycol (EG) (EG 100% and EG 90% with 10% of water). The fluidic oscillator was designed and manufactured with the possibility of connecting pressure sensors with a high sampling rate, allowing the measurement of the dynamic processes inside the oscillator. The experimental measuring circuit consisting of a middle-pressure pump, hose, and fluidic oscillator was equipped with flow and pressure diagnostics. The output profile of the self-excited fluid jet into the air environment was observed using a high-speed optical shadowgraphy technique. The dynamic viscosity values were measured in the temperature interval (18–46 ℃) for selected concentrations of EG. The results of the observation of oscillations occurring inside the fluidic oscillator and at its output are discussed in the paper.