Oven grease is difficult to get rid of and needs a lot of effort. But keeping a pot of hot water and a bowl of ammonia overnight will loosen it and make cleaning easier, according to the One Good Thing By Jillee blog.
Oven-Cleaner Ammonia Water Baking soda Preheat to 200° then turn off. In a heat safe glass dish, fill with ammonia and set on middle rack (about 1 to 2 cups of ammonia). In another dish, fill with boiling water and set on bottom rack (2 cups). Leave overnight. The next morning, remove dishes and wipe down the inside with a hot wet sponge first drizzled with baking soda.