Neither a "buycott" nor a boycott is likely to accomplish much beyond the symbolic. In the first case, the Citgo brand (marketed by Citgo Petroleum Corporation, which has been owned by Petróleos de Venezuela,
the national oil company of Venezuela, since 1990) doesn't have nearly enough presence in the U.S. to satisfy demand; in the second case, boycotting a gasoline brand over political issues is problematic for a number of reasons (not least of which is the notion that threatening not to buy gasoline from someone who is threatening not to sell it to you doesn't sound like an effective ploy for either side).
Although Citgo may be owned by Petróleos de Venezuela, it is a formerly American company which is still headquartered in the U.S. (in Houston, Texas), employs 4,000 people, and supplies 14,000 independent retailers with gasoline and other petroleum products — Americans with no substantive connection to Venezuela who would be economically harmed by such an action. (Citgo also provides free or discounted heating oil to low-income communities and tribal reservations within the United States.) And, of course, in today's oil market Citgo could likely find alternative buyers for its products far more easily than the U.S. could make up the shortfall created by a cut-off of Venezuelan oil.
As we've noted in many other articles discussing various schemes regarding where and how people should purchase gasoline, the global and fungible nature of the world oil market doesn't really provide consumers with many effective opportunities to influence political issues through their buying patterns.