Any virgin oil product or partially refined product, which has not been previously used, which has become contaminated with nonhazardous impurities such as dirt or water, and which has been returned to bulk storage by the product’s manufacturer, transporter, or wholesaler for gravity separation of contaminants, is exempt from this article. Any petroleum product which becomes contaminated with any other petroleum product during refining, transportation by pipeline, or storage and which remains usable as a refinery feed stock or as a refinery fuel is exempt from this article.