Job has had enough of his tormenting comforters (Job 16:2 f.). He could, if the positions were reversed, well enough offer them such mere verbal consolation (the stress in Job 16:5 is on mouth and lips). Translate Job 16:5 as a continuation of Job 16:4. I could strengthen you with my mouth, and my lip's compassion I would not spare (the last clause after LXX).
Related Commentaries of Job 16:1
Job 16:1-5
1 Then Job answered and said,
2 I have heard many such things: miserablea comforters are ye all.
3 Shall vainb words have an end? or what emboldeneth thee that thou answerest?
4 I also could speak as ye do: if your soul were in my soul's stead, I could heap up words against you, and shake mine head at you.
5 But I would strengthen you with my mouth, and the moving of my lips should asswage your grief.