Synthetic Oils

In the research I've found, American Company Amsoil pride themself on the best, best product when it comes to oil, Synthetic oil.

There product remains unavailable here in Australia, why?

Is it because of price? I know the product is expensive. Perhaps there's more to it than I wish to know.

So I use other Synthetic oils in my motorbike, and so far I am quite impressed by Castrol GPS 10w-50.

Any riders out there with any feedback, be it positive or negative, about either Companies or their products let me know what experiences you've had.