After stumbling on this website:
http://www.kyphoto.com/classics/foru...tml?1323826810, discussion with some members and careful reading of other posts, yesterday I tried with vinegar.
The "fog" is almost completely gone (it's visible where it was only if illuminated by direct sunlight).
When I started to brag about my results to my wife, she said "you should have asked
me.. vinegar is the BEST cleaning agent of all, works on everything and it doesn't leave the scratches.."
Looks like fungus in lenses is not the problem any more..