A plugged "hockey puck" (CCV(?) on the cam cover/intake above the exhaust cam) can plug up and overpressure the crankcase causing leaks. There's also a drain hole in the camifold below the hockey puck that can plug.Don't know if yours is a diesel, but my CRD does (or did) something similar - in that the oil thinned out every 3 -6 months and needed an oil change. In my case, over fueling diluting the oil, due to injector blow by - once injectors re-seated, thinned oil issue stopped. It increased an engine main crank seal leak and also caused the engine to burn oil which ceased with oil change.