Hi Irene, you certainly can substitute oil for butter. A few things though: the butter in this recipe enhances the whole flavor of the banana bread. If using an oil use about 3/4 of the amount to the butter, but coconut oil you can use 1 to 1. If you are choosing to use rapeseed or sunflower oil, they are usually good for baking but do have a mild flavor so they might change the flavor of the bread slightly. Coconut oil certainly adds its own flavor so just be aware of that too. Hope it all works out for you.