Melted butter works well when drizzled in during the mixing portion. However, most of the ingredients are cold. If the butter isn't melted, it is very difficult to evenly distribute throughout the batter. Oil is very easy do work with, but has very little flavor. It is used to porvide a moist texture in the final product.
And as for apple dump cake recipes, the one posted here looks like a real winner.
For a change of pace, and when you're in a hurry, an easy, but very tasty apple dump cake can be made by pouring apple pie filling into a 15 inch rectangular cake pan, spreading evenly (this actually works better with 2 cans of apple pie filling). Next, open a box of spice-cake mix and evenly sprinkle over the pie filling. Drizzle 1 stick of melted butter all over the cake mix. Place in a pre-heted 350 degree oven for about 45 minutes. REmove when the top begins to brown aroound the edges. Serve with vanilla ice cream or whipped cream.
Dump cakes in general are easy, yummy, and convenient. Play with cake flavor combinations and the fruits you use. Try cherry, appricot or peach, apple, blueberry (yum), raisins with brown sugar and cinnamon, etc. You cn even make a cheesecake custard and put a home-made or store bought cake mix on top. it's all good.
Seeeeya; Goodweed of the North