Single crystals of furan/phenylene co-oligomers are among the most promising highly-emissive materials for applications in various optoelectronic devices. In this work, we synthesized and studied furan/phenylene co-oligomers with the same conjugated core 1,4-bis(5-phenylfuran-2-yl)benzene and methyl substituents at p- and m-positions of the terminal phenyls. The effect of substituents on the crystal packing, charge transport and luminescence of the single crystals was studied. Compared to the unsubstituted compound, the methyl-substituted co-oligomers demonstrated improved thermostability and enhanced photoluminescence, which we assign to J-aggregation resulting from the strong inclination of the molecules against the main crystal facet. The charge mobility in single crystal organic field-effect transistors decreased upon the inclination of the molecules. We conclude that the molecular tilt angle, intermolecular distances and interactions in crystals of heteroaryl-containing linear conjugated oligomers can be controlled by the introduction of end methyl groups in the appropriate positions.