FORUM RULES UPDATED - The forum rules have been updated to continue to provide a friendly and safe environment for our member community. Please take a moment to review the updated rules and our terms of use and privacy policy. Click here for Forum Rules which has links to the 'terms of use' and 'privacy policy'.
No additive will help with the gunk and water in the tanks. They have to be drained and cleaned. Once you've got fresh gas in a clean tank, seafoam will help keep it fresh and prevent carbon buildup in your engine.
I've found it at Autozone, O'Reilly's and Auto Quest.