No gasket or silicone needed.....the bottom of the sleeve is where the answer lies, It should be a tight fit around the spark plug thread. If it is, take the spark plug out and gently rest the sleeve upside down on a bench, now tap down the lip around where the spark plug hole is. don't tap it too far, go around the lip tapping as you go. Now turn the sleeve over and have it resting on the bench the right way up. Tap down on the lip around the edge of the sleeve......only a little bit again, as you still want it to be a lip, just one that seals around the o-ring.
hope that helps. i've done it many times before and each time its worked. As i said, no goo or silicone needed.