Abstract: Process for treating used lubricating oil to reduce the content of suspended particulate matter therein, characterized by contacting said used lubricating oil with a carbonyl-group containing compound in the presence of an aqueous electrolyte, separating the resulting mixture into an upper layer containing clarified oil and a lower layer containing agglomerated particulates, and recovering oil of reduced particulate content as product. Suitable carbonyl-group containing compounds include ketones, esters and aldehydes and suitable electrolytes include inorganic and organic acids, alkalis and salts.