Ovens naturally create and accumulate dirty grease. The heat and the oven fans, or gas flame, create a draft of hot, greasy steamed air, which is essentially dirty. This is wafted over your foods, and contaminate it. This cannot be prevented, as the very act of oven cooking creates dirty grease and the associated fumes. Dirty oven grease takes a while to be visible, so you can be sure that it is there, well before you can see it.
So when it comes to oven cooking, the main attraction is the food! The food is purchased, the food is prepared, the food is placed inside the dirty oven, and the food is removed once done, and enjoyed. There is no focus on the state of a dirty oven, it is all about the food. Little thought or care is given to the dirty oven.
You can keep your oven cleaner, healthier and happier by simply keeping your oven clean. Your oven cleaner should be able to provide you with a strong de-greaser spray. It will be more powerful than what you can buy in a supermarket. If you can't get such a product from your professional oven cleaner, you can use a combination of bi-carbonate of soda and lemon juice or vinegar. Don't allow it to settle, wipe the loosened grease quickly to avoid metallic stains.
A professional oven cleaner can strip away unseen grease from the inside of your dirty oven and deliver a healthier, tastier meal. A clean oven is also more efficient as it doesn't have to warm up a dirty grease coating.
The main learning point here is to give your dirty oven some love. It doesn't want to be a dirty oven, but cooking at high heat naturally makes it a dirty oven. A good wipe out with Cif or even window cleaner will reduce the dirt inside your oven. Do it, and keep your relationship with your dirty oven cleaner to keep your oven a healthy and happy oven!