Abstract: In a process for reducing the concentration of ash-forming components in a lubricating oil wherein a feedstream comprising said lubricating oil and an aqueous ammonium salt treating agent is continuously passed into a first reaction zone wherein said treating agent reacts with ash-forming components in said oil and an effluent is continuously withdrawn from said first reaction zone and subjected to filtration after removal of water therefrom, the improvement comprising introducing said feedstream into said first reaction zone by passing said feestream through a centrifugal pump along with portions of the contents of said first reaction zone.