I had to replace the shut off valve gasket on my 82 300sd a few months ago and I did not use any kinda gunk sealer stuff, but I did use 2 gaskets and she is bone dry, not a leak at all.
One time though I replaced the turbo drain tube gasket and I used some Indian Head gasket sealer and guess what. Leak. so I removed it all again. cleaned surfaces really good and applied it dry. now no leaks.
hope this helps . zachariah