I'm assuming that the oil was there initially to lubricate the aperture blades, but why does it leak out?
Some lenses suffer from sticky focus/zoom/aperture rings, its completely normal. People and/or service centres often put a little light machine oil or silicon oil inside the rings when re-building lenses.
It's certainly NOT there to lubricate the aperture blades, the last thing you want in the middle of your lens is moving parts flinging oil about!!!
It's pretty much impossible for it to work its way into and onto elements, dont worry, its normal.
The full answer is Patrick's one. I can elaborate a little more, having read some more on photo.stackexchange. The grease used for lubricating the focusing barrel is normally very viscous. However, if the lens heats up, the grease becomes thinner and can flow into other parts of the lens. Therefore, a lens with an oily aperture may have been exposed to high temps (left out in the sun) sometime in its life. There is one lens type - the micro-Nikkor 55mm f2.8 that apparently used a different grease than the other lenses and this grease would cause oily aperture blades and a stuck focusing ring more often than the other lenses.