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My microscope rep worked for Nikon and Zeiss with an independent company, is very trustworthy and he claimed the same thing.  I have never seen it either, but I suspect that he had really did have something happen.  He never told me any sales nonsense at any other time.  In his bad experience, the issue was mixing oils from two companies by not cleaning an objective before putting a different oil on it. I agree it seems unlikely there is a problem, but for safety, given the cost of objectives, I would just clean the objectives before switching oils, and you will not have a problem.  Dave

Dr. David Knecht
Professor, Department of Molecular and Cell Biology
University of Connecticut
91 N. Eagleville Rd.
U-3125
Storrs, CT 06269-3125