[W]ithout China and Russia, this is just a theoretical exercise. In fact, here's a good time-saver: if you read any story about a gasoline embargo on Iran, just scan quickly and get to the part where the reporter explains how and why Russia and China would go along. If it's not mentioned, the story is inconsequential.
Drezner also explains why the recent uprising is likely to lessen any chance of China or Russia going along with sanctions:
I'm betting that Moscow and Beijing have observed the "Death to Russia!" and "Death to China!" chants among the protestors. This is likely going to make them even more reluctant to do anything that undermines the current regime (even if this hurts their long-term interests). Which a gasoline embargo would most certainly do.
Do I think a gasoline embargo is a good idea? Absolutely. Do I think it will happen? No, I don't.
Ackerman agrees, adding:
[W]hy would, say, the United Nations agree to a move that would push the Iranians to dare the international community to confront it militarily over a global economic chokepoint?