A new method to determine by quantitative analysis the amount of dye present in dyed engine oils is provided. The method uses imaging techniques, such as spectrophotometry, and a combination of dilution and standard addition techniques to quantitatively determine the concentration of dye in the engine oil to ascertain whether dye concentrations in engine oil have decreased as compared with original specifications, and to quantify the amount of dye to be added to the dyed engine oil to bring dye concentrations to original concentrations.