Cosmoline is goopy stuff, similar to Vaseline, used for protecting metal against rust. It’s most commonly used on firearms these days.
A similar product called Boeshield was developed by Boeing, and is notable because it comes in a handy spray. Both are quite effective, but need periodic re-application, which would have been impossible in the vauld.
Update: It was too rusty to even attempt to start. 
Damn it.
Is it too much to ask that 50 years ago they knew how to bury things properly?
So does that mean the contest winner is now required to dispose of a useless, rusty ’57 Plymouth?
I bet they could still sell it to a collector.
That’s a shame.
It’d be interesting to see, though, how well we could preserve a car underground for fifty years using today’s technology. I bet it would work a little better.
What make and model would we bury.
I’d go with a Hummer, or some sort of mini-van.
It is interesting how some thing survive much longer than that while buried and others don’t. We have spent 6 years taking down a hopelessly decayed barn built in 1776 and the final excavation work is underway now. The excavation crew found a Massachusetts license plate from 1915 that I originally thought was a replica because the finish was so blue and white and intact. The year is plain as day like the day it was made as are most of the numbers. It is some kind of baked enamel on metal that can survive anything. Sadly, the excavation equipment popped off part of the enamel but you can still read the whole thing and the rest of it is pristine. WWI was going on then and it just got buried to be found 92 years later in almost perfect shape if someone never ran over it. Even still, the untouched parts looked remarkable and the whole thing is easy to read.
The Jesus tomb episode on the history channel blew my mind whether the suggestions are corect or not. It is undisputeded that tomb vaults about 2000 years old can still be found in Isreal mostly intact and can be brushed off and read underground the same way this car can except in better shape. I suppose it is all about moisture.
I think that Doc Brown (Back to the Future III) had the right idea - entomb the car in a low-humidity area, like a salt mine, or in a desert mountain cave well above groundwater level.
The part to remember is that of the eight cylinders, only one or two would at or close enough to top dead center that both intake and exhaust valves would be closed. All the rest will be partially or fully open, providing a passageway for air exchange via the carburetor or exhaust system, such that humidity can freely act on the cylinder walls, creating must rustitude.
Paging Roland LeBay, or Arnie. 
I want to add in something about preservation. The Army Ammo Plant by Baraboo has been getting cleaned up for years, and many buildings are being removed. One of the contractors that were hired to take down a building, found a motorcycle preserved in a pit of oil. It was there for the purpose of keeping it stored if needed, and was eventualy forgotten about until it was removed. It was in great condition, not counting something like the seat.