I've fried bananas in a pot with some butter, but they remain soft and I would like to make them more crispy. Should I use oil instead?
I assume you are referring to standard, grocery store, cavendish bananas-
They have a ton of water and sugar and have a very delicate texture. This makes them easily disintegrate and get gummy instead of crisp. The key is to deep fry them at a fairly high heat until they are dark and crisp. It also helps to use greener bananas that will hold together better.
Butter is not well suited to this because of the necessary oil depth and high heat. Not only would it take a fair bit of butter but the butter would start to burn. Even clarified butter wouldn't be ideal.
Vegetable oil or shortening would be the way to go.
I think you'll have better luck if you twice-fry them. Instead of butter use a vegetable or maybe even a peanut oil. Slick them to the desired thickness and give them a quick fry. Remove the slices and place on paper towels to cool. Then fry again. The first go around seals them and the second will make them crispy.