What to put on my base gasket?

[bandit390]

To help it not leak?

[F-16Guy]

Nothing. Get the gasket surfaces perfectly clean, torque the cylinder properly during assembly, and it won't leak a drop. If you HAVE to use it for some reason, a thin coat of high temp automotive RTV will work fine. Just be sure not to get it too close to the oil passage on the right side of the cylinder, you don't want any RTV to get stuck in the passage and cut the oil off from your valve train. (disregard if you have a 300ex)