I doubt it will rob any more actives than say filtering with cotton or hemp fiber. With that being said I do think that they (cotton and hemp) do collect oil.
So, I would suggest smaller amounts of carbon changed often. Since the carbon will probably become coated in claim. However, since carbon can be used as a lubricant in powder form, there is a posibility that oil will not cling to the carbon at all. Either way I would recomend pitching it and not try reclaiming... Your likely to extract carbon as well with most solvents I imagine...
Wish I had a carbon filter, I got a little here thata little to harsh for me on the nail. But it effects the best from the nail.