dont keep running it that low as it will die, it could be possible at that low of a pressure that it has allready started damaging the shells, get them checked before it spins one i would not be happy with anything less than 20 psi at idle, you usually find tho on rs turbos when the light comes on its allready too late knocking or not.
i suspect oil pump, is it a new build and did you charge the oil pump and pick up pipe with oil prior to starting as many dont and this tends to air lock the pump causing low pressure