Every crude oil has some unique things about it depending on the region and even in particular the area it comes from. How about this idea why not test oil tankers for Isis oil. If found the entire oil tanker and it's oil is seized
This only matters if the receiving party GAS about the political situation, which China and a lot of third world countries don't.
Take a look at this Wikipedia link to the "Oil and Gas Journal" list of refineries in the world and see how many you can count that wouldn't grab cheap oil at the drop of a hat;
LINK Ed