There is more vanillin and less tannins in American White Oak than in French Oak, and it is the charring of these oak barrels that gives American whiskey much of its flavor. Soaking charred oak chips in bourbon can give a hint of bourbon, and the presence of the caramelized sugars also adds to the flavor when whiskey is filtered through maple charcoal in oak barrels.
Used bourbon barrels are often used for aging rum, and you can substitute this for a used bourbon oak barrel by reusing the chips and soaking in aged rum.
The amount of distilled congeners is different for both Scotch and Irish whiskies, and both can be aged by soaking in Spanish sherry or bourbon oak barrels. Scotch whiskey is double distilled, while Irish whiskey is triple distilled in the oak barrels, and it is also possible to soak in aged whiskey a second time to make an essence. You can also soak American oak chips already soaked in bourbon or European oak chips already soaked in Spanish oloroso sherry in oak barrels that contained aged whiskey.
Tennessee and bourbon whiskies don’t use sugar and caramel when aging in oak barrels, although the others mentioned above all do. The style and taste would determine the amount of burnt sugar (caramel ) to use, although a good measurement to start is one tsp /litre.