In actuality you can't boycott Citgo gas. You can't really boycott any oil company or even an oil producing nation. Gasoline and oil products aren't like, say, produce, wherein the different "kinds" can be separated. Oil is purchased by all companies and mixed at all refineries.
You can boycott the stations, but you will still buy their petroleum products at most any other station from which you purchase gasoline. Moreover, Citgo was an American corporation in its former life, still has a base in Houston and employs 4,000 Americans, so the damage that you do would be more to Americans than anyone else.
You can get the full scoop at http://www.snopes.com/politics/gasoline/citgo.asp
, wherein Snopes breaks it down for you.
The petroleum industry is completely inbred and is a nation unto itself, which it is impermeable to consumer buying patterns. This explains many of the Bush Administration policies, from the invasion of Iraq to ignoring the terrorists in Saudi Arabia to the attempt to hand over our port security to a company in Dubai.