An oil composition which has a kinematic viscosity at 100 °C of from 32 to 50 cSt and which contains less than 3%. polynuclear aromatic compounds is employed as a process oil for rubber compounds, especially aromatic rubbers. The process oil is compatible with aromatic rubbers but contains less toxic polynuclear aromatic compounds than conventional aromatic rubber process oil. Rubber products of acceptable quality are produced by the use of the said oil composition.