Brush vegetable shortening with a pastry brush into a 12-cup Bundt cake pan. Flour the pan and tap out excess flour. Set aside.
Separate the eggs, carefully. Do not get any egg yolk into the egg whites or they will not whip. Whip the egg whites with the cream of tartar in a clean, grease free bowl. Whip until you have a fairly stiff peak. Set aside.
In a large mixing bowl, using either a stand mixer or an electric hand mixer, cream the butter until light and fluffy. Add the sugar gradually, beating thoroughly. Scrape the sides and bottom of the bowl with a rubber spatula occasionally.
Add the egg yolks, one at a time. Scrape the bowl again. Add the lemon juice, lemon zest, buttermilk and lemon oil. Combine.
Sift the flour, salt, and baking soda into a bowl. Gently add the dry mixture to the cake batter. Scrape the bowl again, mixing everything thoroughly. You want the cake batter to be smooth.
Gently fold the whipped egg whites into the cake batter.
Bake in a preheated 325 degree oven for about 1 hour 10 minutes. Or until a toothpick when inserted comes out clean.
Remove the cake from the oven and let cool for 10 minutes in the pan. Remove the cake from the pan and let it cool completely on a baking rack. Once cooled, then drizzle with lemon icing.