Magnetic needle interfacial shear rheometers are a valuable tool in the study of the mechanical properties of fluid thin films or monolayers. However, it is difficult to separate the interfacial and subphase contribution to the drag on the needle and, in principle, this separation should be favored by decreasing the needle diameter. Here we show the results obtained when using a new needle type, namely magnetic microwires with diameter approximately 10 times thinner than commercial needles. We show that the lower inertia of the microwires calls for a new calibration procedure. We propose such a new calibration procedure, based on the flow field solution around the needle introduced in Refs. 1,2. By measuring thin silicone oil films with well controlled interfacial viscosities and eicosanol Langmuir monolayers, we show that the new calibration method works well for standard needles as well as for the microwire probes. Moreover, we show that the analysis of the force terms contributing to the force on the needle help to ascertain whether or not the measurements obtained are reliable for given surface shear viscosity values. We also show that the microwire probes have at least a tenfold lower resolution limit, allowing to measure interfacial viscosities as low as 10(-7)N·s/m.