Abstract: A method and apparatus for determining and extending a change interval for an operating fluid of a unit, particularly for an engine of a motor vehicle, uses at least one directly or indirectly recorded operating variable which represents the qualitative condition of the operating fluid. An evaluation of the at least operating variable and a continuous calculation of the change interval therefrom is performed by a computing and control unit. The calculated change interval is related to a specified unit service interval and, if the change interval expires before the service interval, a regeneration signal for the removal of a certain quantity of the spent operating fluid and the replacement of the quantity with a fresh operating fluid is produced as a function of the difference therebetween.