You don't. Tons measure mass, whilst barrels measure volume.
Actually, you can. To convert barrels to metric tons, you have to first know the API or specific gravity of the product. So let's take a...
It already is.
The watt is a metric unit. It is equal to one joule per second. The joule is equal to a newton-meter. And a newton is equal to a kilogram-meter per second squared.