Benzene, toluene and xylenes are mainly produced as by products beside metallurgical coke production. Absorption into the washing oil is of crucial importance in such technology. Washing oil is a complex mixture of hydrocarbons with higher boiling point and the washing property is changing during application period. Process parameter which is monitored as indicator of absorption process efficiency and washing oil condition is mainly concentration of unabsorbed benzene in coke oven gas outlet stream. Secondary parameter is viscosity of washing oil. The aim of reported study was verification of the optimal operating conditions of absorption unit of benzol and identify the ranges of physical properties (viscosity, solid phase content, surface tension and density) which mainly effects the process of benzol absorption. Beside the increasing absorption efficiency is significant extension of life cycle of washing oil, which makes the by-product technology more sustainable.