The most dramatic case was when a friend of mine was told by another garage mechanic to try this oil... it is the best there is. He did not know that I was a dealer at the time, and he took the fellow up on it. My friend had a Buick Grand National (T-type=250 horsepower with turbo). The car only had 25,000 on it. Apparently the car's turbo was already beginning to coke the oil and create sludge. The mechanic gave him the oil out of a 55 gallon drum with no instructions to flush the engine first (of course at this mileage whod a thunk it necessary). Anyway the Amsoil moved the sludge and plugged the oil screen and starved the engine of oil. The car was never quite right after that even with considerable mechanical work, and then was sold.
I know of a number of examples where cars on Amsoil in PA used the oil almost from new and did not last any longer, and some not as long as the dino cars on regular changes. I believe Amsoil makes good products, but I believe the "extended drain promotional" has cost some people extended engine life ( my experience as well with two vehicles). If you used other oils on extended drain intervals, there would no doubt be more problems. I am convinced though that there is always the limit of an oils' life due to the contaminants that accumulate, even if the original oil does not break down.
The bypass filter does not totally eliminate this problem in my view. Amsoil is good, but I think it needs to be changed more often than the marketing suggests, at least in some engines.