Marine journal bearing is so often subject to the risk resulting from the loss of lubricating oil pressure, which negatively affects its performance. The inevitable result of such a condition is the complete failure of the main journal bearing of marine slow speed diesel engine. The study at hand introduces an experimental investigation aiming at better identifying the impact of oil supply pressure in relation to the pressure and temperature distribution within circumferential grooved bearing. Test trials have involved tracing those two critical profiles in case of optimal supply pressure as well as when it is either decreased or increased by 25%. The experimental procedures have also extended to cover the influence of two different oil grades on the outcomes derived for the previously mentioned operational conditions. All of the conducted experimental test trials were carried out with observation to the loading program of marine slow speed diesel engine involving different speeds and loads. It was concluded that maximum oil film pressure is seriously affected when oil supply pressure is decreased especially under high loads. In regard to the oil film temperature profile, it was observed that it is slightly affected under such operational conditions in which speeds are slow and range from 32 rpm up to 102 rpm.