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It's usually due to oil on the aperture blades, which are designed to work completely dry. If you've got it open enough to reach the blades, you can often clean them with just cotton swabs and lighter fluid (Ronsonol). Use it sparingly. The fluid is very thin and wicks, so if you're not careful, you can help it transmit the oily residue deeper into the mechanism, rather than remove it. It's easier if you have the lens sufficiently disassembled to reach the blades from both sides, but if you can only get at them from one side, it's usually still enough. In that case, it can be helpful to hold the lens so that you're introducing the swab from below, to help avoid muck dripping deeper into the lens or onto the glass elements. It's also helpful to twist the ends of the swabs so they're very tight, and not too linty. It sucks to reassemble a lens with a hunk of lint inside.
Go slow, change to a fresh swab often, and you should be ok.