While there is no question that Amsoil is one of the best synthetic motor oils available, you have to read the fine print. The 25,000 mile extended drain interval requires regular oil analysis and filter changes using only Amsoil filters. If the periodic oil analysis (for which a fee is charged) reveals certain levels of contaminants, you must change the oil. Its not like you can just put Amsoil in your car, and drive it 25,000 miles between oil and filter changes.
Mobil 1 used to make similar claims, but was forced to stop after being sued. Personally, I wouldn't recommend going longer than 12,000 miles with any synthetic, unless you increase the oil capacity and filtration efficiency by adding one of those nifty dual Hi/Low bypass units like those sold by Amsoil.
My take on synthetics is that, 20 years ago there was a real difference. Today, the science behind conventional oil blends and their additives have vastly improved to such a point that the typical passenger vehicle engine in good working order will never push a good mineral based oil beyond its protective limits, given adequate drain intervals and proper vehicle maintenance. Of course, if money is no object, and you like spending it, then synthetic oil certainly isn't any "worse" than conventional oil.
Deisel, high-performance, or extreme environment applications are another matter.
On edit: I suppose for the sake of disclosure that I should make it known I was an authorized Amsoil dealer for several months. But, after putting in some considerable research time, I could no longer tout the 'benefits' of synthetics as a sales pitch when the preponderance of available evidence shows that modern conventional oils provide more than adequate protection in most cases. So, I gave up the dealership and when my drain interval came, I replaced the Amsoil with good old conventional Valvoline.
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