Leonard Rynkowski
Leonard Rynkowski voted up Rooster Cogburn's answer

From Betty Crocker :

Q. Can I use melted butter or margarine instead of vegetable oil in the brownie batter? A. You sure can. Using butter or margarine will make your brownies slightly richer with a bit of "dairy" taste. If you use margarine, you'll need to bake the brownies 2 minutes longer.