I'm not sure what kind of oak is used, but a lot of the casks are used ones purchased from American distillers like Jack Daniels. They can be used only once for the American whiskey, so The Scots purchase them and break them down for shipping. From what they said at a whiskey tasting tent, the Scots rent the casks to the Spaniards for aging wine before breaking them down again and continuing their journey to Scotland, thus paying for the original cost of the barrels and the shipping. Leave it to a Scot to figure how to get free barrels.