We report a very high force resolution for tuning fork based Shear Force Microscopy as used for feedback regulation in scanning force (SFM) and near-field scanning optical microcopy (NSOM). The sensitivity and dynamics of fiber tips attached to 100 kHz tuning forks are investigated both experimentally and theoretically. Operating the tuning fork at vibration amplitudes smaller than 10 nm allows discrimination between viscous damping due to capillary wetting, and fiber bending upon tip-sample approach. Force distance curves on hydrophilic sample surfaces indicate the direct transition from non-contact operation to tip-sample contact. Recording resonance spectra during feedback at relative amplitude attenuations of less than 0.5% reveal frequency shifts of less than 50 mHz, indicating the viscous damping mechanism. Finally the viscous damping force acting laterally on the fiber tip is calculated to approximately 100 pN using the finite element method (FEM).