[emeritus]

I use Dow Corning 3145 RTV (to MIL-A-46146 if that means anything to anyone) which is a military-grade non-corrosive sealant. It is fairly viscous (even when fresh), so tends to stay put and not drip or run, and is recommended for use with electrical equipment. I got mine from Farnell some years ago, and although its use-by date of Feb 2014 has long since passed, it is still good to use. Not cheap. Use Meths or IPA for cleaning.

Reputable quality type N and BNC connectors used with a bulkhead socket or feed through should provide effective sealing. They incorporate synthetic rubber seals at both the mating surfaces and cable clamps to prevent ingress of moisture. I believe that the "N" designation comes from it having been designed for Naval applications, although this is only one of a number of theories that I have come across. My late brother-in-law's sea-going yacht used type N feedthroughs and exterior connectors for his external wireless antenna and they never gave any trouble. The connector catalogues I have only refer to sea water performance and make no explicit mention of oils.