I hear alot of claims of amsoil or royal purple or any other synthetic oil giveing better MPG ..... why and how .... anyone find this to be true.

 

less friction.
i havent noticed any noticeable gains after switching to mobil1 synthetic

 

I don't know the answer to your question but I can tell you that I switched over to synthetic (Mobil 1) about 3 weeks ago. I fill up once a week and today when I filled up I checked my mileage and was getting 1 MPG more then before and I haven't changed anything else including my driving habits.

 

You can on average expect a .5 to 1 mpg increase if you use a true, 100% full synthetic instead of a conventional oil. Switching to ALL synthetics everywhere you can could achieve better results.
In September of '07 I changed the differential oils, transfer case, transmission and engine oils and even dumped a bottle of their Purple Ice in the radiator in my Jeep Grand Cherokee to Royal Purple offerings and have enjoyed a solid 2-2.5 mpg increase ever since. (well till I just put a lift on the thing and went to larger, more aggressive tires, anyway)
My truck has NOT been due for major fluid changes, but I use RP in the crankcase. Since changing from Mobil 1 about three years ago. I don't get any real mpg benefits, but it idles a bit smoother and the "Hemi tick" is there, but a lot quieter...

 

Ive used RP forever and its in my engine and trans and Ive never notived any mpg gain unless I keep the RPMs down.