Your Cutting Board was initially finished by hand rubbing 10-12 coats of Walnut oil.
When your Cutting Board begins to lose its shine, pour a small amount of Walnut Oil on a paper towel and rub it into the Cutting Board.
I recommend that you continue to use Walnut Oil to bring the luster of the wood back. You can find Walnut Oil in your local grocery store.
After using your cutting board wash it with dish soap and water, rinse and let dry.
DO NOT PUT YOUR CUTTING BOARD IN THE DISHWASHER!
Bob Livingston - Woodworking Craftsman