Film thickness measurements in grease lubricated contacts are presented for different greases. The conditions used in the experiments are similar to the ones expected in fully-flooded slow rotating bearings. The results show that at very low speeds grease produces film thicknesses substantially thicker than base oil lubrication. An empirical model is developed which can reflect this behaviour. Input to the model is a simple film thickness measurement test to characterise new grease. The model is used to estimate the effective grease viscosity leading to the bearing lubrication parameter kappa. This model is proposed for fully-flooded slow rotating bearings instead of the current practice using the base-oil viscosity.