Buttermilk is an acidic ingredient, which helps break down the gluten in your baking and creates a more tender cake.
Vegetable oil stays a liquid at room temperature, while butter solidifies.
Instant Clearjel is a starch that allows your cakes to retain more moisture. For substitute use Instant Pudding mix as it contains Instant Clearjel.
I suggest scanning cake recipes and choosing one that already contains buttermilk and/ or vegetable oil.
Bake at a lower temperature, and make sure that the temperature is accurate.