Eventual motor failure will occur if the oil level in the mower's crankcase falls too low. A leak in the oil supply--anywhere on the machine--requires prompt addressing. You'll need to make a thorough inspection of the oil system on your Briggs & Stratton lawnmower.
Oil Tank Leaks
Oil originates in the tank and travels via the oil pump into the crankcase, where the crankshaft stays properly lubricated. Before the oil can travel into the pump, it needs to pass through a pickup nozzle inside the tank. This nozzle can get clogged from impurities in the oil, causing a backup in the system and a leak out of the oil tank. Leaks can also occur when projectiles hit tank walls. Drain any oil remaining in the tank into an approved container. Disconnect the oil tank from the engine to disassemble, clean and inspect the tank.
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Leak Near Engine
Another common source of an oil leak occurs near the gaskets at the fill tube and oil sump. This type of leak allows oil to escape from multiple points, as missing pieces in the gasket will allow oil to move into other engine areas. Pull the dipstick out of the tank and inspect the area around the base of the fill tube. If the leak continues, the oil sump needs further cleaning. Replace the gaskets for both the fill tube O-ring and the oil sump.
Leak Near Carburetor
Before it reaches the crankcase, oil must travel through a breather cavity, which has small holes that can get plugged by impurities, causing a backup in the oiling system and a leak near the carburetor and air filter. Remove the breather cavity, which is underneath the oil tank, from the engine. Wipe the cavity clean with a rag and drop a thin, metal wire through the hole in the bottom to push out any clogs. Replace the breather cavity's gasket.
Oil Pump Problems
After the oil leaves the tank, it travels into the pump, where it gets pushed into the crankcase. If the oil pump gets clogged or starts to fail, engine oil may stop moving throughout the system. Backups then produce leaks in a number of areas, including near the air filter. Pumps can fail for a variety of reasons, and they will likely fail at some point throughout the mower's life. To access the oil pump, major engine disassembly is required. Ask a service mechanic to address oil pump repairs.