Abstract: An automatic oil level maintenance system for an internal combustion engine of the type having a crankcase containing a supply of oil and a vacuum source when the engine is running. An airtight oil reservoir contains a supply of make up oil and a vacuum actuated pump has a spring-biased flexible diaphragm therein. The crankcase, oil reservoir, pump, and vacuum are operatively connected by conduits, with the source of vacuum usually the carburetor of the engine. The pump maintains the oil in the crankcase at predetermined engine run and stop levels. The vacuum is applied to the diaphragm when the engine is running and any excess make up oil is withdrawn from the crankcase with the resulting oil level therein falling to an engine run level. The vacuum is removed from the diaphragm when the engine is stopped and make up oil is forced into the crankcase by movement of the spring-biased diaphragm with the resulting oil level in the crankcase rising to engine stop level.