It is a very long process to clean the Aperture Oil in a Pentax Takumar 28mm 3.5 SMC but it is well worth it as this is a great little lens and are easy to find. Just make sure you have a spanner before you start repairing the lens.
Try this before you pull the aperture apart in your lens. It may be a simpler fix in the mount. This is not a guarantee but a quick check. If you see oil on the blades then you will have to pull the aperture. However this might be a simple fix.
This video shows how one can disassemble, repair, and reassemble these Promaster MC and Marexar CX MC 28mm 2.8 lenses. The problem is not always the aperture but more likely just the mechanism in the read mount that gets sticky from old grease.
These two videos show how to disassemble and repair a Minolta 58mm f/1.4 PF MC lens and then reassemble. Some people are converting this beautifully built lens to digital by removing the mount and making modifications as well as adding a M42 Adapter ring. The demonstration is broken down into two videos you can see here and here.