Just means that there is a bit of oil build up, and cause the stock engine have the PCV (pressure crank ventilation) go back through the intake the oil gases eventually build up. Probably worth putting a catch-can on, that will remove most of the oil before go back into the intake. Check how much oil you use over a 1month period, if it's a very small amount then don't worry, if it's 1/2L or more then more than likely you'll have broken ring-lands.