As always, use 91% isoproply rubbing alcohol to clean each surface, use white paper towels and keep scrubbing the surfaces until the paper towel no longer picks up dirt or oil.
IF you want to use red RTV sealent, apply it on by finger, Don't squeeze it out of the tube.
You only need to barely "skin" the gasket surfaces. I would do this merely to hold the gasket in place "not" as a sealant.
A properly cleaned and dried oil free surface and a new DRY gasket will seal up perfectly every time.
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After more reading, it seems common to have these adapters crack and or warp.
As a preventative measure, use some 2k grit wet dry paper on a piece of glass and "surface" the gasket side so its smooth and flat.
That will ensure a perfect gasket seal.
Always surface parts by sanding in a figure 8 pattern with light pressure and alot of soapy water.