I got a free copy of the [Nikon 200mm f/4 Ai lens](http://www.photosynthesis.co.nz/nikon/ai2004b.jpg).

However the aperture blades are stuck and oily from both sides. I managed to open the lens and clean the blades from both sides with alcohol. I reassembled the lens and its works just fine. However the oils seems to come back over time and the blades are stuck again. I disassembled the lens now for the third time; cleaned the blades and reassembled everything. It always works for a few days but then the oil comes back and glues the aperture blades back together. How can I permanently fix this issue?

Since it was for free I don't want to bring it to a service center and pay a lot of money for it (maybe I would let them fix it if they would fix it for 20$ max). I would prefer to fix the lens on my own. I'm interested in learning how to fix it.