The use of wax-crystal modifiers for improving low-temperature operability of diesel fuel has not been possible in the United States because the large crystals could not penetrate the very fine porosity of fuel filters. The development of new, extremely potent additive packages has solved this problem by reducing the size of the crystals precipitated in the diesel fuel. The smaller size allows them to pass through the fine filters of auto-diesel equipment at temperatures well below the cloud point, even under extreme field conditions. On the basis of successful testing of additive-treated diesel fuel, cloud point can no longer be considered as an indication of operability limit in auto-diesel equipment. It is desirable, therefore, to develop a laboratory flow test that predicts the field performance of diesel fuels-flow improved or not-and accept it as the basis for a new operability guideline for diesel fuel equipment.