Oh, this tired old wive's tale again. *sigh* Will it never die? Probably not.
Dielectric grease is MADE FOR CONTACTS. That's it's whole purpose in life.
Actually, the purpose of dielectric grease is to seal and lubricate spark plug boots.
Applying it to electrical contacts relies on a mechanical wiping action squeezing the grease out when the connector is mated. In reality, when used with a low-pressure connector, what happens is that the high spots are (hopefully) pushed through the grease film to make metal-to-metal contact.
Basically, the same thing happens with electrically conductive grease, such as Kopr-shield. The difference being that the copper particles fill in some of the microscopic scratches so the metal-to-metal surface area is greater.
What difference does that make? To a hand-held ohmmeter, virtually none. The meter has a small battery and a high impedance and does not impose a significant load. As power goes up, the difference becomes measureable.
Rather than rely on internet gurus, I prefer to look to the manufacturer. From a Permatex document:
"Dielectric grease, or tune-up grease, is a silicone-based grease that repels moisture and protects electrical connections against corrosion. It is also used to keep dirt, water, and other elements out of electrical connections. It has many uses, including automotive tune-ups, home electrical work, and recreational vehicle wiring. Dielectric grease does not dissolve in most liquids, so it is good for marine and outdoor applications. You can use this grease to lubricate the rubber parts of electrical connectors, spark plug wires, and to protect electrical connections on your boat or recreational vehicle from corrosion. Since dielectric grease is a silicone grease, it should not be used on silicone-based rubbers or plastics, as it will break them down over time. The grease does not conduct electricity, so it shouldn’t be applied directly to the mating surfaces (pins and sockets) of an electrical connection. "
Your tractor, your choice.
In any event, neither will remove oxidation. My first recommendation, De-oxit, will remove oxidation and leave a protective layer. I back that up with dielectric grease applied around the connector shell, including the back side of unsealed connectors.