While doing a short test flight, I noticed an oil pressure drop. Since I was close to the airfield, I landed immediately. On the ground, a major oil leak was noticed: oil poured out via the air intake.
The problem was only a blown gasket: the gasket between the engine casing and the oil sump. Since I landed immediately, I lost only 3 to 4 liters of oil, so the engine never ran "dry". No further damage to cylinders or pistons.
Check your engine and see if this gasket is not leaking. With the pressures inside the engine, this gasket wil be blown out sooner or later.