An improved system for automatically continuously replacing an engine's used lubricating oil with fresh lubricating oil throughout operation based on engine operating conditions is disclosed. The present system includes an intermittent injection control device for intermittently injecting small amounts of used lube oil into the engine's fuel system. An electronic controller is provided to vary the amount of used lube oil injected into the oil system based on the severity of engine operation. The system periodically determines an engine operating severity value, i.e. fuel consumption value, for an interval of engine operation and calculates a base line quantity of oil for injection based on the current fuel consumption of the engine during the current interval. The system may also include an auxiliary flow control device for directing controlled quantities of fresh oil into the engine's lube oil system for maintaining the engine's oil sump at a proper level. The system maintains the quality of the engine lube oil at a level necessary to provide optimal engine protection at all engine operating conditions while also maintaining the oil concentration in the engine's fuel at an acceptable level.