I'd be very careful if I were you. Sure, there is a great chance that the oil and filter will stand up to Amsoil's change interval. But all they will clearly state regarding voiding manufacturer warranty is that the oil itself meets or exceeds specifications, thus will not void the warranty, but does not clearly state that extending the manufacturer's change interval won't void the warranty.
Besides, as I have been saying for quite a while, it isn't just the oil breaking down that is a problem, but combustion blowby contamination. Nobody has been able to answer to my satisfaction as to why contamination is not an issue. I understand oil breakdown perfectly, but even oil that stays brand new will get contaminated.