The base oil used was PAO4 from ExxonMobil Chemical (Houston, USA) and the OFMs were GMO and GMS supplied by Sigma-Aldrich (Madrid, Spain) with a high purity grade (> 99%). The steady-state friction coefficient was determined averaging over the last 10 min of the experiment. Three repetitions were used for each OFM and concentration to reduce the noise in the HFRR data.