Organic field-effect transistors based on aligned small molecule semiconductors have shown high charge carrier mobilities in excess of 10 cm² V⁻¹ s⁻¹. This makes them a viable alternative to amorphous inorganic semiconductors especially if a high reproducibility can be achieved. Here, the optimization of high mobility organic field-effect transistors based on the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b] benzothiophene (C8-BTBT) via the addition of a polymer additive to the printing solution is reported. Specifically, films and devices are compared based on solutions of the neat semiconductor and the blend with polystyrene and shear-coated devices with excellent device characteristics and gate-voltage-independent mobility values reaching 12 cm² V⁻¹ s⁻¹ are shown, which are the highest reported values for C8-BTBT-based films prepared by a scalable, solution-based process.