I have no doubt that Amsoil products are good. As for worth the cost.... not too sure on that one. I have taken several commercial trucks to over 1 million miles on so-called regular HDEO's. One, I took to 1.4 million and it still got better than industry average mpg and only used a gallon of oil every 7,000 - 8,000 miles, and was never been opened up for more than the overhead being run and one replaced injector. And it got 30,000 mile oil changes using Kendall 15w40 and did not use a bypass filter. I have not ever seen one truck go substantially further on engine life by consistently running synthetic. And the so-called "case study" articles put out by Amsoil are subjective at best. And I don't fall for the wasting oil scam. Virtually all used engine oil is recycled. Several companies put out very good oils themselves that are made primarily from recycled oils. Northland comes to mind.
I will contend that extending OCI's could make it more worthwhile, but that is all relative. How long can one extend the drain? Each situation would be different, as there is so many variables that can contaminate engine oil and cause breakdown. All it takes is one bad air filter to skew up the extended drain plan. Not counting the myriad of other small component related failures that could occur to mess things up. Is there any possible fuel mileage savings? Hard to quantify since there are so many variables that affect fuel mileage, and it has been shown that when someone does a mod or changes oil brand, etc, that they also tend to be more observant of fuel mileage and they operate the vehicle with more awareness of fuel mileage. So the mpg saving could be more related to the driving than the oil.
And considering the additional cost of the synthetic (and shipping if it isn't locally available), especially for someone like myself that uses 11 gallons in just one engine, it hasn't shown to be that good in relation to cost. Not when I can get a good quality HDEO, delivered to my house, at a total cost per gallon of right at $11. And when one is doing the extended drains, they will still be repacing full flow filters and usually be also using a bypass unit. That adds to cost.
When it comes to a pickup truck used to pull a bass boat on the weekend, ROI may mean nothing. But when equipment is used in business, ROI is everything. And I do use synthetics in other component applications like trannies and diffs. I can get substantial drain intervals using them there (500,000 miles), and the OEM also will extend the warranty by 50% (to 750,000 miles) if I use synthetic in those components. So that is cost effective. Engines.... not convinced.