Good Morning Tuned RX300. I agree that viscosity is the #1 characteristic in any lubricant. Specifically in ATF, a high viscosity index (VI) is about the same for synthetics and mineral oils (around 180). I'm not picking on Amsoil specifically but any oil company that misleads the public into thinking that their oil meets all the specs considering that Mercon V, Honda Z1, Dextron III and Toyota T-IV are incompatable in many respects. In the case of Toyota T-IV, ATF's role is friction control so you have better clutch engagement. Your typical "Universal ATF" doesn't address Toyota's clutch material. One way to achieve this is to add lubegard additive to enhance engagement. Most quicklubes use this and carry only one ATF for inventory purposes.

http://www.lubegard.com/automotive/trans_atf_hfm.html

If you insist on using Amsoil or any other non T-IV ATF, use it in conjuction with lubegard to maximize performance - or - save the trouble and use T-IV. The following link was found that might shed some more light on the topic.

http://www.tundrasolutions.com/forum...tranny-cooler/

 

For fluids that meet spec(s), there is no need to add Lubegard.
I raised this possibility that Aisin Warner may very well be the transmission supplier for multiple manufacturers in another thread.
Aisin's web site list their powertrain products for a list of transmission customers, with Toyota as the biggest customer of all.
http://www.aisin.com/product/auto/drive/index.html
http://www.aisin.com/profile/customer/index.html
I found out about Aisin because right on P258 of my RX300 user's manual, in a foot note and bracket (anyone wants to guess why?)

is mentioned right after

This thread is very useful when I tried to search for a failed tranny from an Amsoil user. I tried to do the same thing on my Acura forum. I could not find a single case. Now I have accumulated 2000 miles on both my Acura TL and RX300, both transmissions shift better than w/ OEM fluids.
There is one guy with ES that has 25K on Amsoil ATF with no problem.
https://www.clublexus.com/forums/maintenance/91626-t-iv-synthetic-now-available.html
Here is a group buy thread, it is hard to believe all these Amsoil users are dealers.
https://www.clublexus.com/forums/sho...2&page=1&pp=15

If you have oil analysis result to show how Amsoil's publication is wrong, please post.