Abstract: An improved method and system for automatically continuously removing an engine's used lubricating oil and injecting the used oil into the engine's fuel system throughout operation based on engine operating conditions is disclosed. The present system includes a lube oil replacement pump system for injecting small amounts of used lube oil into the engine's fuel system and recycling controlled quantities of removed used oil into the engine's lube oil system. The process 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 lube oil pump system may be in the form of a compact, unitized lube oil replacement pump assembly integrating the various components of the system.