I'm not sure if it's true but I wouldn't be surprised if it was. In many countries, oil is handled mostly by multinational corporations that do business with other multinational corporations across borders. So much oil is moving that it's impossible to track where it comes from or where it's going. Also, most of the time, the same companies that drill the oil also refine it into oil-based products. They also trade oil on the commodities exchange. Governments might not know (or care) for decades if anything illegal is going on. Government leaders who get too curious about their oil get replaced.
TLDR: It's impossible to know. But the oil market is so huge that it's very easy to sneak in some terrorist oil here or there. All oil looks the same. All money looks the same.
Added bonus: Public transit isn't going to save the world from terrorism. Oil is used in so many things like plastics and fertilizer. It's pretty much impossible to escape. Sleep tight!