Adam Clarke on Mark 6:13

Anointed with oil many that were sick - This is only spoken of here, and in Jam 5:14. This ceremony was in great use among the Jews; and in certain cases it might be profitable. But in the cases mentioned here, which were merely miraculous, it could avail no more of itself than the imposition of hands. It was used symbolically, as an emblem of that ease, comfort, and joy, which they prayed God to impart to the sick. For various examples of its use among the Jews, see Lightfoot and Wetstein on this place.