WD-40 should work, but Thread Penetrate is that much better.
Spray it on the bolt, let it soak for 15 minutes and come back with a 1/2'' breaker bar. Depending on the length of that bar, it'd probably make it much easier for you to use a piece of metal tubing over that breaker bar and use it as a cheater bar.
The more length to the bar, the more leverage you have and thus more torque you're putting down. Only downside to the length is you're probably going to have to take off the top radiator hose to get some room to turn the socket. All you really need to do is break it loose intially 1/4 turn and it should be loose enough from there to either loosen by hand or a regular 1/2'' socket wrench.