I have never heard about amsoil either... is it that good as you say it is? I got the mobil 1 because i was told by many people that it can push to around 17,000-20,000km if just for normal use and i wanted to see if thats true. But on the other hand you wouldn't want to leave oil in there for that long...
I wouldn't be testing that out, mate. The oil may be up to it, but the filter won't be. Mine gets swapped out every 7,500km - in other words, at scheduled services and at the half-way point. Day to day driving on a standard car, something like Edge will be fine - if you're running mods and putting the car under higher-than-average stress levels, I'd be spending a bit more on a group IV as suggested. AMSOIL is quite hard to locate around Sydney, I've found. You can pick Royal Purple up at any Autobarn.
As suggested, the most important thing, regardless of which oil you're using, is to keep it changed regularly, with a good filter. RP Filters are very good, as are K&N oil filters - but if your car is still under warranty, then run with the genuine filter. It's about the only thing that isn't hugely over-priced from Holden Spares!