I won't know this. I never had any problem with my len yet. (keeping my fingers crossed.) Maintain your len regularly. Even if you are not using, make sure you take them out once every few months to clean with blower and check for fungus. After which keep them in the drybox. After every shoot do not keep your len immediately in the drybox. Let it rest for a few hours or a day, clean the len and exterior with a good blower and a clean piece of silk before keeping them into the drybox.
As for servicing in camera shop, I don't have any experience regarding this. Anyone care to share your expertise?