Tectonic faults are often lubricated by viscous fluids which can have different natures e.g. gas, water, brine, melt and viscosities varying over 7 order of magnitude (from 10-4 for water to 103 Pa s-1 for a melt at high temperature). Moreover, understanding fluids viscosity effects on fault dynamics can shed light on induced seismicity in engineering reservoirs where fluids with viscosities ranging from 1 mPa s-1 to 1000 mPa s-1 are also injected during hydraulic fracturing process in order to increase the rock permeability. Here, we examine the mechanisms coming into play in presence of viscous lubricant film between 2 rock slip surfaces during 4 creep experiments performed with a rotary shear apparatus on samples of Westerly granite increasing step-wise the shear stress on the simulated fault.