Years ago (mid 80's), I had a good friend who drove a Pontiac Phoenix (X-car) for about 100,000 miles - on Amsoil - and NEVER changed his oil!! The car was eventually totalled in an accident, somewhere around the 230,000 mi. mark, if memory serves, and still only consumed a neglible amount of oil. The key to his results, supposedly, was the use and regular cleaning/replacement of an add-on external filter that used a roll of TOILET PAPER!! Pretty neat, actually. Apparently, the synthetic base stock molecules are extremely resistant to thermal/shear viscosity breakdown, and as long as you keep the abrasive particulants filtered out, the oil itself should be good for an extremely long time (indefinitely??). Although, I'd be worried about corrosion inhibitors (acid neutralizers) wearing out.
The high mileage/leaking thing is a result of oily plasticizers migrating out of rubber seals in the presence of synthetics - makes seals shrink slightly and reduces their elasticity/flexibility. Not as much of a problem with mineral oils, as its chemistry is closer to the plasticizer itself. This was a problem early on in the use of synthetics, but from what I've heard in the many years since, additives (seal softeners/swellers) are now almost universally added to sucessfully deal with this worry. Not sure about Amsoil, in particular, but I'd imagine so.
I've been a bit curious myself about Amsoil. The last few issues of MCN (Motorcycle Consumer News) included an exhaustive test of a large number of cycle and auto oils, and Amsoil consistently rated near the top (in lab tests). As did Mobil 1. Considering cost and availability, I'd probably go with Mobil 1, myself. Amsoil appears exellent as well, though.
Just don't run ANY synthetic in a leaded fuel burning engine - period. Trust me. I've seen first hand the engine damage that resulted from Mobil's disastrous entry into (and subsequent swift exit from!) the synthetic aviaton piston engine oil market. Mobil AV-1 - ouch!
Bob