Dielectric grease for 30 amp plug and receptacle?

You won't hurt anything using dielectric grease or cosmoline but won't help anything either. Deoxit works for cleaning but won't help if the socket is burned or mechanically compromised.

Turn off loads and pull the CG breaker when plugging in or unplugging.

When a CG socket gets burned and loose there's nothing you can do about it. If there's a 50A socket available use a dogbone and plug into it