Properly functioning stern tube seals will keep the oil inside the stern tube and water outside the system.
But under certain conditions a little oil consumption or water ingress may occur.
Big fluctuations in water pressure on the AFT seals can be a reason for this.
Below video clearly shows how these fluctuations in pressure can occur. One moment the complete stern tube is out of the water, the next moment it is 5 to 6 meters below the surface.