When the ingredients used as a cement are bituminous in character, in alternates the same general type of bituminous cement shall be used in both the patented and proprietary pavements and the unpatented or nonproprietary pavements, the only variations permitted in this particular being the natural variations which are inevitable in the proper fluxing and blending of cements of a bituminous character for distinctive types of pavement, or where the bituminous cement is subjected to mechanical oxidation and to the impregnation of filler or other fine material by mechanical means subsequent to the original distillation.