Snopes debunks the "Boycott gas for one day!!!" e-mails currently circulating. Simply not buying gas on a certain day doesn't do anything to oil companies - you're just shifting the day you buy gas. The same amount of gas is consumed either way. In fact, if you're hurting anyone, you're hurting the independent gas station owners who have nothing to do with the price of gas.
Bottom line: if you receive any e-mail asking you to do something that sounds kinda suspect, check Snopes. They know all.