Abstract: The low temperature filterability of a petroleum middle distillate fuel, such as a heating oil or a diesel fuel is improved by adding to the fuel a flow improving amount of wool fat and, if desired, a flow improving amount of wool fat in combination with oil-soluble, auxiliary flow-improving material such as: an alkyl aromatic, e.g., the condensation product of chlorinated wax and naphthalene; a polymer of an acrylic ester, such as polyalkyl acrylate; etc. This invention is particularly advantageous in improving the flow of a diesel fuel through a fine filter at low temperatures.