Years ago an old Norwegian steam fitter name Olie (true) gave me a tube of Moly Kote Valve Seal by Dow Corning. They used it on assembly of extreme high temp/pressure valves. I have used it for over 30 years and never had a disassembly problem. It is not a sealant but a super assembly lubricant. This stuff is good for O-rings, valve stems, almost anything in the plumbing department.
Dow Corning Molykote 55 O-Ring Grease Off-White 150 g Tube