A method is provided for reprocessing a petroleum-based waste oil feedstock into diesel fuel. The method includes forming a treated feedstock by (a) filtering the feedstock, thereby removing solids and metals from the feedstock, and (b) dehydrating the feedstock; vaporizing the treated feedstock to produce an oil vapor; passing the oil vapor through at least one catalyst bed and subsequently through a cooler, thereby converting the oil vapor to a hydrocarbon liquid product with a diesel product boiling point range; and removing contaminants from the hydrocarbon liquid product, wherein the contaminants are selected from the group consisting of particulates and color precursors.