Actually, the AMSOIL ATF will last at least twice as long without oxidizing than will the non-synthetics.
In fact, even though the manufacturer's talk about 60K and 100K drains, the ones who benefit from this are the transmission shops.
As a practical matter, most good technicians recommend fluid change about ever 15K for front-wheel drive and 25K for rear wheel drive.
I've been using the AMSOIL ATF since it first came on the market. In my Dodge van, I ran it about 150,000 miles. Before the rusting hulk got towed away, I pulled an oil sample. It showed all was well, except they flagged my copper, which was 1 ppm over the recommended level for a 25,000 mile drain interval.
As a practical matter, AMSOIL ATF is, for most personal vehicles, a lifetime fill. For commercial operation and/or towing, it is better to err on the side of changing more often. If one uses oil analysis, then the interval can be extended indefinitely. As I recall, some NC school busses were getting about 80,000 miles on a load before requiring replacement. I think they changed to the Torque-Drive fluid and were getting over 100,000. Their previous drain interval was 20-25,000 miles, best I recall.