Thanks everyone for your responses, my question has definitely been answered!!!

When I have used oil in lieu of butter in brownies, I have also used cocoa rather than melted chocolate. This seems to solve the potential "greasy" problem. But I much prefer the brownies made with butter.

They might end up too oily-- I usually decrease the oil amount by 20% when subbing in for butter since butter is only 80% fat. Maybe add milk or water for the remaining 20%

I have definitely used oil in brownies instead of butter. Depending on how much it calls for, I would slowly drizzle oil in the melted chocolate and whisk to combine.