The oil filter housing gasket (OFHG) is the #1 oil leak on E90 N52 engines. The rubber gasket shrinks from heat, allowing oil to seep. Oil contaminates the serpentine belt and can damage the alternator if ignored.
Symptoms
Oil leak from passenger side of engine, oil on serpentine belt, burning oil smell, oil dripping near alternator
Fix / Remedy
Replace OFHG and all associated O-rings. Clean oil from belt and alternator. Replace belt if oil-soaked.