Mike,
Forgot part of your email. Yes I believe that full Synthetics yield better mileage, but since it is one the order of 1 or 2% many don't see it. I put my cars on it quickly because I believe that, more importantly than mileage, the reduced wear & tear are worth it. On my Suburban I changed it every 3,000 miles. Well more than required but good for wear & tear reduction. I ran the numbers: it ws cheaper to change every 3,000 than to put in a rebuilt engine. ...And at 381,000 my OEM engine still gets 5,000 mies per quart.
So as I see it there are two things: 1st reduced wear & 2nd better fuel mileage. You decide which is more important. Like I said: I keep cars for the long haul. It saves money.
Bill

Mike,
I find Mobil One ATF (I don't think "Mercon" applies to M1) at AutoZone. They seem to carry it consistently. Other places you have to look around. ...And Sam's Club/WalMart just don't carry it. Its big advantage is that you don't change it as much... so they don't get the turnover they need to stock it.