I have been using AMSOIL products for almost 5 years and would not use anything else. I use the motor oil according to their recommendations, filter change at half-cycle (6 months or 12,500 miles, whichever comes first) and oil and filter change at 12 months or 25,000 miles (whichever comes first). I keep detailed records of fuel mileage and have seen a 10% average MPG increase in my vehicles. I'll be using the Series 2000 0W-30 motor oil in our Pilot as soon as we get it broken in.
AMSOIL had the first API approved synthetic motor oil in 1972 (4 years before Mobil 1). They have an excellent reputation in the lubrication industry even though they do no major national advertising. Family owned and operated for 30 years.
Be careful. Most of their oils are not API approved. Amsoil itself recommends various oils for specific API applications. Read the words carefully.
I received a response about this from Amsoil. They did not deny this. They gave me a lot of double-talk, including something about a disagreement with most of the automotive industry about acceptable levels of certain chemical...I can't remember what it was (I'd have to go look at the response.)
Be careful of companies who proclaim their product is the best. They pay those "independent" research companies in used in their adds. Do you think Amsoil has the money to pour into research that Mobil does?
A few years ago, Consumer Reports did a study where they rebuilt Chevrolet engines used in Taxis in New York. Wear parts such as pistons, inserts, crankshafts, etc where numbered, measured, and in some cases weighed. These engines then underwent various oil change intervals with different brands of oil.
You should read the article to see the specific results. It should be available online for subscribers at www.consumerreports.org or possibly in your public library.
The gist of their article was that any oil meeting the API recommendations for a gasoline engine was OK regardless of brand. They commented on synthetic oils and felt that they were better for extremely harsh environments that almost no US drivers face. They suggested that oils are changed because of unfilterable contaminants, not because they are worn out. The use of a superior and longer lasting oil should not change the need to replace this oil once contaminated beyond the filtration capabilities of your filter.