Metal conditioners (MC) are added to lubricants to enhance their friction and wear in friction pairs, mainly in engines, gearboxes, and rolling bearings. Its growth in the Brazilian market is primarily focused on internal combustion engines. The effect of mixing MC with commercial engine oil (SAE 5W-30 API SN) was studied regarding the rheological and thermal properties. Also, the tribological performance of steel–steel contact was investigated. The rheological and thermal properties were determined by flow curves (at 20, 40, and 100 °C) and differential scanning calorimetry (DSC), respectively. Reciprocating fully-lubricated tests were performed at 40 °C and 80 °C (Po = 1.7 GPa, 5 Hz). Differences in the chemical composition between SAE 5W-30 and its mixture with MC were identified by infrared spectroscopy and related to their tribological performance. The coefficient of friction remained within the range of 0.09–0.1 for all conditions, typical of lubricated steel–steel contacts under boundary and mixed lubrication regimes. However, the mixture improved the wear resistance by around 33% when lubricated at 80 °C compared to the wear resistance offered by 5W-30. The formation of tribofilms with different chemical compositions was confirmed by SEM-EDS for all conditions. At both temperatures, the tribological performance reveals beneficial synergy between the metal conditioner and fully formulated oil additives. The tests lubricated with the mixture at 40 °C showed a less severe wear mechanism when compared to the tests lubricated with neat 5W-30. The study demonstrated that the mixture maintained the physicochemical properties of the commercial oil with a substantial anti-wear action at 80 °C.