Self diffusion of base oil in six model-lubricating greases and in the base oils themselves were measured by pulsed-field gradient nucleic magnetic resonance (NMR) at 23, 40, 70 and 90 °C. Three greases were based on naphtenic mineral oils and three were based on synthetic polyalphaolefin oils. Soaps of 12-hydroxy stearic acid were used as thickener agents in all six greases. The ratio between the diffusion coefficient of the base oil in grease and neat base oil in itself were evaluated as function of temperature. We judge this ratio as a direct indicator of the obstruction effect that the thickener has on the oil. The thickener showed a stronger obstruction effect on the base oil in the synthetic grease than in the mineral based. This is related to the soap content, which for these greases, is roughly twice as high in the synthetic greases as in the mineral ones. The obstruction effect is constant in the temperature interval 40-90°C for mineral- as well as synthetic based greases. We judge NMR to be a promising method for investigation of how the gellant affects the diffusion coefficient of oil in a lubricating grease.