Can't comment on Amsoil, but can comment on extended oil changes running full synthetic. Have had two high mileage E-class Mercedes, factory recommended oil change is 10-12k using Mobil 1. Had 220k on one vehicle before it was totaled, engine still ran like a top. Have 170k on the other one and it runs like new. Had an 06 cummins that I went 15k on oil changes 7.5k on filters using mobil 1, at 150k it ran like new.
I have no qualms with extended oil changes running a quality full synthetic.
