The tribological properties of the magnesium composite alloys reinforced with solid-state synthesized Mg₂Si or Mg₂Si/MgO dispersoids are evaluated under wet conditions (in engine oil lubricants) by using pin on disc type wear test equipment. Every composite shows a dependence of the friction coefficient on the applied load, which corresponds to Sribeck diagram based on elasto-hydrodynamic lubrication (EHL). The increase of Mg₂Si content causes the increase of both the friction coefficient and the total wear by plowing, because Mg₂Si dispersoids protruded from the surface are much harder than S35C mild steel counter material. When including MgO dispersoids formed in deoxidizing SiO₂ particles by magnesium, the friction coefficient reduces to 0.01–0.02 in oil lubricant. This is due to the “mild offensive effect” by MgO dispersoids which ease the attacking or plowing phenomenon on the counter material, because they are not so hard.