Oil-changing system for an internal combustion engine

A method for maintaining the quality of lubricating oil in an internal combustion engine having an oil reservoir comprising periodically removing used oil from said reservoir in an amount no greater than 20% of the total capacity of the reservoir, and simultaneously adding to said reservoir a substantially equal volume of fresh oil, the rate of said removals and additions being substantially equal to the engine manufacturer's recommended rate of complete oil change based upon engine usage factors, said removing and adding of oil being effected at opposite extremities of said reservoir and accomplished by electrically operated pumps capable of generating a significant output pressure while preventing backflow, the operation of said pumps being dictated by monitoring means which record the extent of oil usage, and said removed used oil being transferred to a container having an exit drain.