I finally found some Nylog and plan on using it soon. I got the HFC type even though I will be using it on a mineral oil system. I read that it is semi compatible with mineral oil. I don't plan on getting it inside the system anyways.
Something I've never heard anyone say about Nylog is that it should be used WITH thread tape on NPT fittings. This is from the instructions on Nylogs website.
I use Nylog on flares and gaskets and a few other things like assembling txv's. For NPT fittings I use leak lock, it always seems to work just fine. Follow the instructions and allow it to set for a few minutes before assembly and I've never had an NPT fitting on a refrigeration system leak.
He was talking about the brass 1/4"SAE access port fitting that screws into the head of most large filter shells, not the shell seal itself. Even though I personally put a small film of nylog on the new cover o-ring gasket before I bolt the head back down.
I just have made it a personal habit to use it on any gasket, O-ring, any threaded item, valve caps, or whatever it may be that is trying to prevent refrigerant from sneaking out. Stuff is truly Nectar from the Gods.
I'm talking about the threaded hole in the cover where you can install a access fitting. I have seen at least 20 shells with leaks around the access fitting that had Leak Lock on it.. I usually remove it, run a tap down the threads and install a new fitting with Leak Lock. Sometimes it seals and sometimes it doesn't.
All of the "leak stoppers" have their ideal application like y7turbo said.
I carry and use at least 3:
1. Nylog for mating surfaces.
2. True Blue for pipe threads.
3. Leak lock for leaking glands,"pack them glands and pack em good boy".
4. lock tight for SAE/metric threads