Stribeck curves were obtained from numerous traction tests performed under a wide range of operating conditions on several fully formulated gear oils with mineral, PAD and ester basestocks. These curves, which make use of a modified Stribeck parameter also presented here, were examined in order to analyse the influence of the operating conditions and of the gear oils on the coefficient of friction in mixed and boundary film lubrication. Particular attention was given to the coefficient of friction in boundary film lubrication. Theses Stribeck curves might be helpful in understanding the evolution of the coefficient of friction between gear teeth along the meshing line.