Well, in all the years Amsoil has been marketing, they have had virtually no warranty claims relating to their 25k recommendation, providing the engine was otherwise in good mechanical condition, in particular, without coolant ingress, head gasket leaks, malfunctioning PCV, etc.
Another question to perhaps ask, is 'how does Amsoil know that the oil can't go twice as long as 25k miles' with that engine/filter/oil combo? After all, you wouldn't want to be dumping out $10/quart Amsoil prematurely, now would you?
I question their advice with respect to filter changes personally, more than their advice with respect to oil changes. In my opinion, their recommendations of premature filter changes places them in the same league as the 3k mile/3 month quickie-lube promoters. Many modern vehicles specify filter-change-intervals far in excess of 12.5k miles.
used oil analysis is ultimately your best guide, and I think you will generally find the BiTOG crowd to be enormously conservative with respect to oil changes, even with used oil analysis data. A used oil analysis taken close to when the engine is coming off of warranty is a fairly good idea anyways -- much better 'cheap insurance' than a gazillion excess oil changes.
Another paradox to consider with Amsoil -- only people who care a lot about their oil will generally buy their product, the rest of us just use quickie-lubes or a favourite dino/synth brand. Those who care a lot about oil tend to change frequently. I can count on one hand probably the number of 25k mile OCI used oil analysis I've seen here with Amsoil -- so not a lot of people actually follow the recommendations, which keeps any potential warranty claims low, allowing Amsoil to keep reinforcing that 25k miles is okay, and the cycle continues.
