1 cup vegetable oil
½ cup white or cider vinegar
1 tbsp poultry seasoning
½ tbsp Italian seasoning or oregano
½ tbsp garlic powder
½ tsp salt
¼ tsp black pepper
That's the basic recipe. You can jazz it up with other things too; sometimes I add beer, some rub, bay leaf, etc., plus always toss in some parsley for color.
You can marinade for an hour or overnight, take out and put on a hot grill. You'll get flaming, have some water nearby. Turn then baste, letting the egg congeal before turning again. You'll build up a crust on the chicken with the basting. Cook until 160° in the thickest part (or higher, whatever your preference).
The firemen would build a pit with interlocking sides about 3' high and a bed of charcoal about 4' wide and 12' long. They'd use large 4'x4' wire mesh hinged double grates with long bars on each end so they could open the grate, lay down the chicken halves, close up and lay across the pit, then one fireman on each side could flip the entire thing to cook the other side. The juices and marinade would drip down into the coals, emanating the most wonderful aroma you could smell throughout the entire town; you knew it was field days time!
Try it, you'll like it!