Experimental study of the 79VKG-25/5 screw compressor was carried out in compressor mode on industrial oil with the addition of Polyisobutylene (PIB) at the following solution concentrations: 0.2%, 0.4%, and 0.6%. The productivity of a screw compressor practically remains constant in the range of changes in PIB concentrations from 0 to 0.2% and decreases on average by 10%. The decrease in performance is due to an increase in internal gas flows in the screw compressor. From the presented dependences it follows that in the entire range, the introduction of PIB additives does not lead to a significant increase in the air temperature at the outlet of the screw compressor and, consequently, in the pressure in the working cavity of the compressor. Therefore, the main reason for the increase in leakage and a decrease in productivity, apparently, is the deterioration of the sealing properties of the working fluid in the gaps between the housing and the rotors. The results of experimental studies are presented in the form of graphical dependences of the consumed power, volumetric productivity and isothermal efficiency of the 7VKG-25/5 screw compressor on the degree of pressure increase and polyisobutylene concentrations - in the range for the working fluid flow rates V(M1) 100 l/min and V(M2) 140 l/min.