I'm referring to the grease/lube that comes applied to the contact area of new circuit breakers, Is it dielectric, or is it conductive?

I doubt it is conductive. The contact pressure squeezes it out but it does slow oxidation of the area that is not directly metal to metal.

Some derivative of anti-ox?

I think it is just grease so the breaker seats easier but it will prevent some level of oxidation.

Thinking back, I remember a tech at a Bell Labs spin-off giving me a squeeze tube of 'lube' that he said was conductive, and 'expensive'. I used it on a few 200/400 amp disconnects. It looked like the stuff in the pic above. Afterwards, he said the tube was around $50, this was back mid 80s.

Reminds me of the “light bulb grease” sold at auto part stores. Or, even, the “ dielectric grease” sold there.

It’s been my experience that dielectric grease, despite the name, does nothing to prevent conduction by direct contact. It will bliss corrosion and prevent arcing through gaps (such as between a coil wire boot and a distributor hub).

I have seen grease with silver particles suspended in it used to lubricate tuning hardware in broadcast transmitters. $60 for a 6.5 ounce tube. Then there's the copper anti-seize grease used on spark plugs.