Is whisky matured in a sherry cask Kosher? I know there is a Machlokes about this. Who holds what?
Essentially Rabbi Moshe Feinstein does not seem to have a problem with whisky matured in sherry casks. However he is reported to say that a "Ba’al Nefesh" should be stringent.
There is a very full discussion here where you can find all the sources.
Amongst Kashrus authorities, a search showed
2) cRc - Liquor List - Chicago Rabbinical Council Similar to 1
My personal experience is that most people including our LOR are not worried about whisky matured in sherry casks.
There's a good article that summarzies the opinions here: http://doseofhalacha.blogspot.co.uk/2013/07/kashrus-of-scotch.html Read through the comments, too:
Rabbi Moshe Feinstein was addressing blended whiskey, but the issues are basically the same with regards to a sherry-cask scotch as far as I know.
Rabbi Feinstein said it was allowable, but not preferable; he personally made reasonable attempts to avoid such a product (as a chumrah) , and gave his blessing to those who would make a product certified as free of non-kosher wine. So authorities such as the cRc and Star-K who don't recommend "sherry-finished scotches" are following Rabbi Feinstein. They're not necessarily saying they're certifiably not kosher, just that they're not endorsed.
The Star-K's position, for instance, is if it says on the label "this contains sherry" (in one language or another), then they don't recommend it. If you don't know, then you can rely on its permissibility if it happens to be there.