Fluids are pervasive in the Earth's crust and saturate fractures and faults. The combination of fluids and gouge layers developing along faults can generate fluids of different viscosities. Such viscous fluids were found to influence the reactivation, frictional stability of faults, and eventually the dynamics of propagating earthquake ruptures. We reproduced laboratory earthquakes on analog material (PMMA) to study the influence of viscous lubricant on fault frictional stability, rupture nucleation, and propagation under mixed lubrication conditions. Experiments were conducted in dry conditions, and with fluids presenting a viscosity ranging from 1 to 1,000 mPa.s. Through photoelasticity, high-frequency strain gauge sensors, and accelerometer measurements, we obtained new insights about the influence of lubricant on a characteristic nucleation length, rupture propagation velocity, and local slip and slip rate evolution during the reproduced frictional ruptures. Our experiments show that the presence of a lubricant generating mixed lubricated conditions along the fault induces, (a) a reduction of the frictional resistance, (b) an increase in nucleation length, (c) a decrease in the fracture energy. In addition, the larger the viscosity of the fluids, the larger the reduction of frictional strength and the increase in the nucleation length. Moreover, ruptures occurring under mixed lubricated conditions showed a pulse-like rather than crack-like behavior, suggesting that viscous lubrication can induce the transition from crack-like to pulse-like rupture along natural faults. We demonstrate, supported by existing theory, that this transition is mainly governed by the stress acting on the fault at the onset of nucleation, which is drastically reduced in presence of a lubricant.