Even though there are some of you who might think differently, you can end up using too much oil in your popcorn machine. But, if the unit is designed to be used with oil, you need to use it as doing so without can really compromise the final results. Also, butter and shortening are not a replacement for oil as they have differing melting temperatures.
Anyway, back to the question at hand. There actually is a recommended amount of oil you should use and it will be based on how many kernels you place inside. For example, if you use ¼ cups of kernels then you should go with around one tablespoon of oil. Then, for every extra quarter of kernels, add another tablespoon of oil.