Special condition additives such as Cold Flow Improvers, Deicer, Cetane & Octane Improvers, Biocides and more are available. City Oil Co. Inc. Provides Premium Diesel fuel products, pre blended with additive, often used during the winter months to protect the fuel against cold weather.City Oil Co. Inc. Also Offers Diesel Emissions Fluid / DEF. DEF is not a fuel additive, but a chemical sprayed within the exhaust of a Diesel vehicle. It is used to break down, and reduce emissions. It is required by law in all in all new Diesel vehicles. Small quantities and larger quantities are available.