Understanding fluids viscosity effects on fault dynamics can shed light on the induced seismicity in engineering reservoirs where fluids with viscosity ranking from 1mPa s-1 to 1000 mPa s-1 are also injected during hydraulic fracturing process in order to increase the permeability. Here, we examine the mechanisms coming into play in presence of viscous lubricant film between the rock slip surfaces during both earthquake nucleation (slip rate from ms-1 to mm s-1) and propagation (mm s-1 to m s-1) where mixed lubrication and fully lubricated regime might be activated, using rotary shear tests on precut samples of Westerly granite.