The thread below from @Will L. really should be a Reference Library sticky. It's (Highly valuable content on/for this site) an in depth discussion that starts with recommendations, but, hopelessly buried in the 6.5 section to where even Google chokes hard and pukes up a lot of irrelevant stuff in finding it. 
Head studs - what will stop the leak
After speaking and Emailing with Permatex: Best: Optimum Black -27037 #2: The Right Stuff (black) #3: Ultimate Black #4: Optimum Grey #5: The Right Stuff (grey) #6: Ultimate Grey There is one other possible option, but it has not been properly tested, even by Permatex for it- Seal & Lock...
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