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.
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.
This Nikon Nikkor 28mm f/2.8 Ai is a lens definitely worth looking for! With a cheap Ebay adapter it can be put on many DSLR’s. The lens produces amazingly sharp images. Most people want the Ai-s version but miss out on the optics of this amazing lens making it a very cheap 28mm 2.8! The focus is absolutely amazing and everything works great.