An oily aperture is not uncommon in old lenses. Typically this happen when old lubricant breaks down due to heat or age. Symptoms of this condition are generally over exposed images when not shooting at maximum aperture (smallest f number). This happens because the blades move slowly (or not at all) when you take the photo. You can usually see the oil on the blades if you look into the lens. Also if you manually open the aperture it wont "snap" back when released. This is why some used lens ads will say "snappy aperture" meaning oil free. There was an interesting discussion here about cleaning the aperture blades on a Minolta 50mm lens. This particular lens is easy to work with and there is a low risk of damaging the lens elements if you are careful because they are sealed in an inner tube. The article leads you here which has a series of photos on this job.  ...