I have a 150XL which I bought used and was fine. I have been overseas and so it has not been used, but stored with the shutter exercised every 2 months. I noticed on my last trip back home that the lens had developed a kind of faint whitish grey residue inside the front assembly. It is most definitely not fungus (which is random and snakes all over the place as expected of slime mould) but as if a fine powder had been brushed very gently in whisps on one of the internal elements. It seems to follow the perimeter contour of the lens in whisps that looke like they have been brushed on ever so gently kinda like the stuff they use for fungerprints.
If my memory is not failing me, there were reports of some Schneider lenses having faulty (cement?) which caused a residue to settle on the elements, which could be cleaned off with an internal clean at Schneider. Can anyone comment further on what this could be before I ship it to schneider? Yes, I am terrified as this is the most expensive lens I own and I have not even used it! I hope to *&&^%^%*& it is not a paperweight now