Oil is a liquid, so fairly "simple" to detect and include, but natural fuel requires strain vessels and has no scent - so it could be hard to contain and difficult to detect leaks in the extraction process. http://Www.Fat-Tgp.com/cgi-bin/atx/out.cgi?id=62&trade=https://images.google.td/url?q=https://atavi.com/share/wvr5qgz1u1e8c