Ultra Grey is a stiffer cure and oil resistant, just not as much as black. Black for anything that holds oil, or dissimilar metals - example Aluminum to steel . Grey is good for intakes, water pumps, or similar metals Aluminum to aluminum. OH, the New Optimum is now easier to remove.
Use any of the Permatex sealers that say "adhesive silicone" or Adhesive Sealer" not their silicone gasket makers I've used all the Permatex Adhesive sealer colors as well as their clear, they all work fine for me
I use Ultra Grey on everything. Zero leaks ever. If it's sheet metal use a gasket as well. That way you can disasemble the parts by tearing the gasket. Doug
I use the Ultra Copper (because I have a bunch of it) and have never had any issues. I think the main thing is to make sure both surfaces are clean, oil free and dry.
I've always had good results with the red HI-Tac used with a gasket. This on both oil and fuel sealing. Not sure I would use it on anything to fill a gap though. I hate the thought of a glob of anything getting loose and wandering around like silicone.
Right Stuff !!! works great ! a few years ago I had slight oil leak under the manifold above the timing chain cover .. Right Stuff sealed it right up !