Aperture blades are generally not meant to be lubricated. The lubricant in a lens is usually grease placed on helicoids or other mechanical parts such as the focusing mechanism. Over time—especially with age, heat, or previous servicing—that grease can thin out and migrate into nearby parts of the lens, including the aperture assembly.

So if you see oil near the aperture ring or oily aperture blades, it’s more likely migrated grease than something intentionally applied to the blades. Heat can make viscous grease flow more easily, which is one reason older lenses may develop this problem after years of use or storage in warm conditions. Some lenses are also known to be more prone to it because of the type of grease originally used or because of how they were serviced.

In short: the oil didn’t start on the aperture blades; it usually came from lubricated mechanical parts and moved there over time.