Hmmm ... tricky. OK, usually, for analyzing the metal components of used oil, the oil is digested with strong acid to dissolve the metal, and analyzed by ICP, or other more archaic methods. For example, military applications may have standard practices that use 'arcs and sparks'. Again, the metal content is analyzed by its own thermally excited fluorescence or optical absolution. You really want to analyze the metal, not the oil.