I am currently using an STP air filter, and I am wondering if this is the right air filter to use to maximize my fuel mileage? Does the air filter make that much of a difference on fuel mileage?

As long as your filter is clean, the brand doesn't make a very big difference.

A air filter can make a small difference in fuel mileage. There are other factors which are more important to take care of are making sure your tires are properly inflated, removing any excess weight from vehicle ( things you keep in your car that don't need to be there,) keeping the air filter clean( replace within manufacturer's guidelines), keeping the car properly maintained including a tune-up, and oil change as recommended by the car's manufacturer, among others.