Pseudo-Chrysostom — as excerpted in the Catena Aurea on Mark 6:6-13

“(Vict. Ant. e Cat. in Marc.) Or else, that it might be a witness of the toil of the way, which they sustained for them; or as if the dust of the sins of the preachers was turned against themselves. It goes on, And they went and preached that men should repent. And they cast out many devils, and anointed with oil many that were sick, and healed them. Mark alone mentions their anointing with oil. James however, in his canonical Epistle, says a thing similar. For oil both refreshes our labours, and gives us light and joy; but again, oil signifies the mercy of the unction of God, the healing of infirmity, and the enlightening of the heart, the whole of which is worked by prayer.”