That may be aftermarket rustproofing -- but it sure sounds like the original Cosmoline (cosmolene) that the factory used to undercoat the cars to prepare them for their overseas shipment. The dealer used a HOT powerwasher to remove the goop when the cars arrived --- it was part of the pre-delivery preparation and inspection. At this age I doubt hot water would do the trick -- but you might try it -- gallon after gallon of really hot water. In the end, you may have to use some mineral spirits -- kerosene -- something like that. Don't get excited and scrape as you'll damage the finish underneath -- try a couple of solvents and this stuff should dissolve and become easy to remove. What it'll do to your garage floor, I don't know! What a mess! You know, you might even try the quarter car wash to remove it before or after a mineral spirit application.