Thread tape, dope, magic jizz, GM seal tabs, blah blah blah.. or just let it drip unit cooling system debris stop the leak.... Other than leaking, that's your only issue.
Note: single wire sensors use the mechanical bond to the head/intake/etc to make the ground path. Chitloads of tape or dope can compromise your electrical connection and skew or kill the readings. That's why I use Yamabond.
...on the knock sensor, DO NOT put ANYTHING on the threads. It needs a specific load to do it's job (doesn't hit water) and any think that will muffle frequencies will alter your knock sensitively to the bad.