3MP,
Please note the phrase: "When used in mechanically sound engines"....
On the rare occasions when I run into serious problems with customers using Amsoil, it generally turns out the engine is running very poorly and dumping all sorts of contamination into the oil. This includes things like clogged PCV valves, fouled plugs,manfunctioning oxygen sensors, and EGR valves that are stuck open with carbon deposits. Once in a great while I'll run into a coolant leak, but those tend to be fairly obvious.
Folks who aren't dilligent about engine maintenance are very poor candidates for extended drain synthetic lubes. OTOH, if you keep the vehicle tuned up and running well, you can use the extended drain Amsoil protocol and still get very low wear rates and excellent engine cleanliness. I've got about 500k of total experience doing this on my personal vehicles over the past 25 years with 0 oil related problems.
Tooslick