that's not entirely true bill.

While synthetic oil does indeed allow for longer change intervals, it has other benefits as well.

For one, it holds it's viscosity spec longer. As oil gets older its viscosity numbers creep up, esp the first/W number. So a 5W-30 oil might end up more like a 15W-30 oil by the time you change it, which is not good, esp in the winter when you NEED a low pour point.

Also, synthetic oil doesn't 'boil off' as much, which is why KlausC always recommends using Mobil1 to folks wishing to avoid PCV system work. It doesn't evaporate as much and doesn't create as much vapour and thus the vapour capture system stays clean.

I agree with you completely that it's silly that Amsoil won't allow their oils to be properly tested. I know of at least one 'homebrew' test that shows how effective it is, and also how it is different than Mobil1 in many ways. This is a facinating study to read:
http://neptune.spacebears.com/cars/stories/oil-life.html

results pages:
http://neptune.spacebears.com/cars/stories/mobil1.html

http://neptune.spacebears.com/cars/stories/amsoil.html

There are many good sources of information about fully synthetic oil out there. I actually think the info on the Mobil1 webpage is quite good:
https://www.mobiloil.com/USA-English/MotorOil/Synthetics/Mobil_1_Circle_of_Performance.aspx

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1998 V70 AWD Turbo 195k+