Jim may well have a point, still my humble advice is to leave sleeping dogs to lie. A spot of what appears to be oil on one blade of the diaphragm does not suggest disassembly. If the lens is functioning well (that is the diaphragm operates smoothly and freely), and the lens is clean and fungus free, leave it be. Now should you need to clean the blades, Jim's advice is excellent. Be careful with acetone, though. Methanol, isopropanol, and ethanol are excellent cleaning agents that will not harm anything in your lens ... acetone isn't always so benign.