Are garlic butter and garlic oil interchangeable?
I agree that they're similar, but I don't think I'd personally call them "interchangeable" in a culinary sense, just like butter and oil aren't really interchangeable in all cases.

Now, it's not like the ingredients police will arrest you for using one in a recipe instead of another. Note that melted butter is about 20% water (depending on how long you've heated it), but you're probably not baking with this stuff (though it occurs to me that it'd be an interesting addition to a salty cracker recipe).