Of all the saddest things of mice or men ... the saddest are these ... the lenses that have been massacred by constant, knee-jerk cleaning.
Occasionally,one comes across them in the "Used" areas, Beautiful Leica and Hasselblad lenses - with a central, translucent spot...
I worked, in a different life, for a company involved in the production of *Highly* sophisticated optics. I saw one on-the-spot firing: against all his training, this guy "cleaned" a lens he thought to be "dirty" with a pencil eraser. This lens was one with a special - I think it was eight- layer coating, which was immediately destroyed.
My best cleaning advice is **DON'T**!!. It takes a LOT of "dirt" to affect lens performance. What will be impacted most is contrast. If at all possible DON'T clean that lens!! Whatever - "Pens", "Miracle Cloths" - Grinding Wheels and Drano - comes in contact with that lens (actually the coating - much softer than the glass itself is most vulnerable), will have an adverse effect - to some degree.
Put "UV" or "Clear" (there's a difference?) filters on the lens, and leave it there wherever possible. Clean the filter if you must - even the most expensive Heliopan is less "dear" than the lens.
We were allowed the use of *one* "system" - distilled water and surgical cotton - with *no* pressure on the cotton. If that did not remove whatever was on the lens - it went to Technicians specifically trained in cleaning - which was most of the time, anyway.