Nowadays, attention to the environment is increasingly emphasized by the scientific community. Different measures have been adopted to ensure a lower environmental impact and increase the sustainability of industrial processes. This work fits well with these principles, as it focuses on the determination of pollutants in waste lubricating oils derived from the industrial and automotive sectors to quantify their concentration for potential recovery and subsequent reuse. Modern waste oils, especially those derived from industrial processes, contain pollutants and metals (most importantly, silicon). It is of utmost importance to determine the silicon content in the collected waste oils to verify the possibility of recycling them since high concentrations can badly affect the industrial operations in which they can be involved. One of the most applied analytic methods for controlling the content of metals is the ASTM D5185-18, Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). However, this method was certified for 50 mg/kg of maximum silicon content. To extend this range, CONOU, because of its duty to monitor waste oil quality, organized an interlaboratory comparison to verify the applicability of ASTM D5185-18 and also to samples of used oils containing higher concentrations of silicon. This work describes the details of the employed methodology and the results of the interlaboratory comparison that showed the applicability of ASTM D5185-18 also to samples containing silicon concentrations higher than 1000 ppm.