A process for re-refining used lubricating oil (ULO) having thermally unstable additives. ULO is heated by mixing with superheated lubricant boiling range hydrocarbons recovered and recycled from the process. The mixture of ULO feed and superheated hydrocarbons is charged to a vacuum column, producing an overhead vapor free of unstable additives and a residual fraction, containing additives. The overhead vapor is condensed to produce an overhead liquid of lubricant boiling range hydrocarbons, a portion of which is recovered as a product and a portion of which is recycled. The overhead liquid free of unstable additives, may be superheated without fouling to produce superheated fluid which can heat the ULO feed sufficiently to permit fractionation. Superheating may occur in a fired heater or preferably in a heat exchanger to prevent high temperatures and cracking of recycled liquid. Mixing of superheated fluid and ULO feed may occur in a pipe in fully developed turbulent flow, and preferably via an inline mixer. Energy efficiency is improved by heat exchanging ULO feed with product streams.