It seems out of the 3 transmission fluids that Amsoil ATF has the lowest Viscosity Index.
1) Amsoil ATF 168
2) Mobil 1 ATF 176
3) Royal Purple ATF 210
I have used:
1) Amsoil ATF
2) Redline D4
3) Mobil 1 ATF
The Amsoil ATF made the transmission shift the best, period.
In cold weather the other ATF's gave me a lazy delayed shift, the Amsoil ATF shifted in the cold weather like it did when the car was totaly warmed up.
I have an Amsoil Dealer in my area that has a 1986 Chevy Astro Van that he used Amsoil ATF in, he just had the transmission rebuilt a year ago, it had 397,000 miles on it when it went in to be rebuilt. These Astro Vans did not have the greatest transmissions and many of them did not last past 150,000 miles.
I talked with a guy who sold Royal Purple and he commented that he had a customer who used Royal Purple ATF in his Chevy Astro Van, and it lasted until 265,000 miles before it was rebuilt.
Of course we have to take into account how both vehicles were driven, the guy using Royal Purple in his Astro Van could have had a heavier right foot than the guy who had Amsoil ATF in his Astro Van.
I have read through all of the posts in the ATF section and there is a guy who has been using Amsoil ATF since 1980 on his various cars and he has never had to replace a transmission or do any kind of work on them.
I feel what is more important than the argument of whether Amsoil ATF is better or not better than Royal Purple ATF is too have a powerful rare earth magnet in your transmission pan, or if your trans pan is aluminum a Magnefine or external transmission filter.
The last thing is to keep the fluid cool with an external transmission cooler if you feel your trans fluid is getting too hot.