oil leak caused by carter ventilation?

[tomvdrunen]

i have a ford transit MK1 2400 Diesel with a big oil leak.
i noticed that the oil comes outside of the oil dipstick (when the engine is running (when you are driving)

so there is overpressure in the carter?
can this be the carter ventilation?
where and how is this mounted? what could i clean?

the engine have engough power (driving is oke) and there is no coolant loose.
(so it's not the cylinderhead or piston rings i suppose.