In the present paper we investigate the influence of tensorial viscosity on the linear MHD spectrum for isolated twisted magnetic flux tubes surrounded by vacuum. The magnetic flux tubes are modelled as 1D cylindrical plasma columns. Viscosity is described by its classical tensorial form with compressive and shear viscosity. The equilibrium parameters of the flux tube are choosen to model coronal loops.\newline Numerical simulations show that in ideal MHD a poloidal magnetic field can destabilize the magnetic structure. They also show that viscosity can strongly destabilize the magnetic structure. This destabilization depends on the poloidal component of the magnetic field.