Hi Peejay,
What you are experiencing isn't uncommon and can be a normal symptom under certain circumstances. The hollow spindle on a quick release Version hub uses a soft silicone rubber seal internal of the hub to allow internal pressure to release, usually caused by varying changes in ambient temperature. The main cause of excess oil breathing through this seal is an over healthy amount of hub oil. The breather is situated within the axle itself so would never let all of the oil out by the very fact that it can't...
Sometimes bringing your bike in from outside to a warm inside or vice versa this can produce these symptoms quicker. If your certain that it's only coming out from the center of the hollow axle tube then I think your quite safe to say your doing ok.
It's also fair to say to may see some oil 'breathing' around each side of the hub, again quite normal. Anything more than this may need further investigation.
Hope this helps.
King regards,
Dave.