IT has been suggested that the use of synthetic oil lubricant may lower the energylosses in large bearings. Such claims are often based on the interpretation of temperature meas-urements in the lubricant and/or bearings of heavily loaded rotating machinery. Lower tempera-tures have been observed in synthetic oils as compared to mineral ones even when the bearingsare operating at the same film thickness. This has been attributed to intrinsic lubricating propertiesof certain synthetic oils.In the present paper temperatures and film thicknesses in large bearings in a hydroelectricpower station were monitored and recorded simultaneously. The results for a synthetic lubricantand a mineral oil were compared. The temperature readings were indeed rather different - that ofthe synthetic oil was several degrees C lower than for the mineral oil. Both had been recom-mended for the actual application, giving virtually identical film shapes and thickness in the twocases.A study of the behaviour of the bearings and the temperature differences, based upon Rey-nolds’ equation and simple heat balance considerations, showed that the lower temperature of thesynthetic lubricant was caused by its higher heat capacity, but gave no indication of the existenceof a special lubricating property. IT has been that that use of synthetic oil lubricant may lower the energylosses in large bearings. The claims are often based on the interpretation of temperature meas-urements in the lubricant and / or bearings of heavily loaded rotating machinery. Lower tempera-tures have This observed by synthetic lubricating as compared to mineral ones even when the bearings are operating at the same film thickness. the has been attributed at intrinsic lubricating properties of certain synthetic oils. the present paper temperatures and film thicknesses in large bearings in a hydroelectricpowerpower station monitored and recorded simultaneously. The results for a synthetic lubricant and a mineral oil were compared. The temperature readings were indeed quite different - that of the synthetic oil was several degrees C lower than for the mineral oil. Both had been recom- mended for the actual application, Implicit virtually film shapes and thickness in the twocases. A study of the behavior of t he bearings and the temperature differences, based upon Rey-nolds’ equation and simple heat balance considerations, showed that the lower temperature of thesynthetic lubricant was caused by its higher heat capacity, but gave no indication of the existence of a special lubricating property.