do not apply di electric grease before you make the connection!
it is a very good insulator and will make your problems worse.
smearing it on after the connection is ok if you want to. you still need to use an inhibitor to protect the connection.
checking for voltage drop when corrosion is suspect is always best done with a load on a line. many times a volt meter will not detect a series fault because the line loss without current is very small, often less than one hundredth of a volt.
you could use a hair dryer to load the circuit, checking it at the same time with your meter. if you have a problem it will be very noticable then.