Here's a nice "money quote" from it: The addition of extra ZDDP usually results in reduced cleanliness, higher engine temperatures and more deposits. Some studies have shown that going past 1400 ppm of phosphorus will increase wear over the long term, and going above 2000 ppm will begin to break down iron and result in camshaft spalling.
I thought Amsoil and its extended drain oils were snake oil. But I took chance and still didn't trust them. (…) I had it tested by an independent oil lab, repeatedly. It went 18,000 miles before the lab recommended changing it.
I believe you. And I did exactly the same thing with exactly the same results using regular ol' normal ol' boring ol' a-third-of-the-price-of-Scamsoil-ol' synthetic Pennzoil on sale from the parts store.