Understanding macroscopic behaviors of suspensions of active particles is a challenging issue in the study of living fluids. We investigate the response of shape-change driven microswimmers to an external flow. In particular, we calculate the viscosity of the dilute suspension of active microswimmers when the flow timescale is close to that of the swimmer activity, resulting in an ample contribution of the swimmer activity to rheology. We find that the shape activity leads to either an increase or a decrease of the effective viscosity, depending on the shear rate. These opposite behaviors occurring for the same microswimmer originate from a phase-locking phenomenon between the swimmer shape oscillations and the applied flow. A simplified analytical model remarkably reproduces the results of full simulations, and offers a promising framework for the derivation of macroscopic constitutive laws for active matter. © 2019 American Physical Society.