On my non-lifting days I am trying to figure out what kind of cardio to do. If i run on the treadmill for half an hour, lets say, whats the optimum speed to run this at to minimize muscle loss and maximize fat loss.

I have seen many champion the HIIT cause. Whats the best machine to run this on (the elliptical that has no arm swing is my guess). Can HIIT be run on a treadmill? Whats the best duration/rest?

Thanks guys.