I was just about to make a cake and I am short some oil, I need about 1/3 cup of oil and have just over 1/4 so could I just add some melted butter or even shortening instead to make up for what I don't have??

For that small of a volume, I think you'd be fine with melted butter.

I agree with karateka, butter will be fine

Definitely. I sometimes substitute melter butter for oil.

Well, just like karateka mentioned, in small volumes there isn't any huge difference.

Wouldn't apple sauce work too?