Take off the muffler guard, and muffler. Behind the muffler, you'll see a small rectangular plate screwed onto the side of the engine. This is the access to the valve springs (don't mess here, not the problem), but the plate is also the engine crankcase vent! This cover has a thick cork-metal-cork gasket that has failed. When that gasket leaks the oil dribbles out right where you pictured it. Easy job to do yourself and just a few bucks for the gasket. Be sure the engine (NOT the mower) model numbers are correct so you get the correct gasket. The engine numbers are under the plastic decorative cover on top of the engine, stamped into the fan housing. Happy mowing!