I bought the lens on a certain online auction site recently and received it in the mail today. It initially looked very clean (it was advertised as "like new in box") until I worked the aperture ring and noticed the oil on the blade.
I'm mulling over whether I should go through the hassle of contacting the seller about returning it (in my stupid haste I left positive feedback after doing an initial inspection of the lens but before noticing the oil).
In the alternative, I could try opening up the lens and cleaning it as Jim Noel suggests. That's something I've never done before and, frankly, I'm not exactly mechanically inclined. On the other hand, I didn't pay much for the lens, so it's no great loss if there are extra screws left over after I disassemble it and put it back together again.
Lesson learned about the perils of that certain online auction site, I guess.