We used coconut oil and you can hardly taste it in the muffins. But you could use other oil. Olive oil can give the muffins a slightly herbal note although it will still be barely noticeable. You could also use vegetable oil, as it has a neutral flavor but the usual vegetable/canola oil is very processed. If you want to use vegetable oil, ideally go for cold-pressed sunflower oil or grapeseed oil, if available.