I loved all the answers and settled on canola oil. Cake turned out to be quite tasty. Thank you.

Definately use the clarified butter! Better yet, brown it...unless it's a white cake!

Yes, you can substitute butter for oil. I've done it many times. It sometimes results in a different texture or flavor than intended, but it has always worked. To be safe, if you've never made this recipe before, use the canola--it's a perfect substitute for vegetable oil.

If you'd really, really rather use the butter because you're thinking it would enhance the flavor of the cake, clarify it first. Then you won't have to do all kinds of calculations to account for the extra water that butter contains. Use the clarified butter measure-for-measure for the oil.

You can absolutely use canola oil in place of vegetable oil; I actually use that for most of my cakes. Melted butter is also an option, though it will change the density and crumb of your cake, so just be aware of that.