Not taken as argumentative....it is a personal decision. However, given the total miles I'm getting with my cars, it can't be that critical.
That is a substantial increase in mileage you got by switching, but it goes against my experiance and every (independant) study I've read....but I can't argue with your own experiance. If I got anywheres near that increase, any increase, I would most definitly be using synthetics also.