Probably not a problem with the hub, they’re not totally sealed and will leak some, more in higher temperatures and/or with low viscosity oil. There are a few Schlumpf hub discussion threads you might check out:
Schlumpf hubs: general discussion.
Schlumpf hub setup discussion thread
Schlumpf hub servicing
Schlumpf oil
You can also get more of the grease that Schlumpf uses.