It is generally recommended to use oil rather than butter in a popcorn machine. Oil has a higher smoke point than butter, meaning it can be heated to a higher temperature without burning. This makes it a better choice for making popcorn, as it can help to ensure that the kernels are evenly cooked and do not burn.
However, if you prefer the taste of butter, you can add a small amount of melted butter or margarine to the oil before adding the corn kernels. This will give the popcorn a buttery flavor and can help to keep the kernels from sticking together. Just be sure to use a high heat-resistant oil, such as coconut oil or canola oil, to pop the corn!