I've been watching this thread with great interest, both from a general perspective and how this applies to any AT, but also from a specific application point of view. I also have a vehicle with an AW transmission, the U151E, in a Toyota Sienna. This vehicle does some summer towing duty at the vehicle's rated maximum of 3,500 lbs.
I am well aware that the T-IV/3309 fluid is nothing spectacular, either in its composition or performance. At the same time, this fluid is not expensive and does the job adequately.
My previous experience with ATs and off spec fluids however have me gunshy to try anything but specified. In a 2002 Protege specifying M-V, I had a change done using Mercon V, not knowing there was a difference. Tranny failure some time later with a subsequent owner whom I know. It wasn't until years later when I discovered M-V and Mercon V were not the same did I make the connection of the tranny failure to the fluid.
Prior to that, an iffy lube changed the ATF in a 95 Dodge Colt (aka Mitsubishi Mirage). My belief is they used a Dex II or III. I first connected the subsequent failure to either their flush machine or iffy techniques, but now wonder if spec fluid might have been ATF+3, ATF+4 or an SP-II or SP-III.
Tranny fluid in both cases were changed at a reasonable interval. I know that was not the issue.
I also hear of differences in seal materials, friction material, etc, from AT to AT. I see transmission models where older units specified one fluid type, and newer units upgraded to another fluid, or the same tranny in different manufacturer's vehicles specifying different fluid. No interchangeability or compatibility is ever stated.
How then can I, be reassured, when choosing a fluid for my application that it truly won't cause harm, accelerated wear, or other negative effect compared to spec? How are the differences in materials addressed? And how can some of these span several specs (I have a theory here, but won't share until I hear an "expert" weigh in).
I have an application where the fluid specified works and is reasonably priced. It is not like a case of Z-1 Honda fluid where the quality is questionable or a case of a ZF5HP19 tranny with outrageous pricing on spec fluid. I don't have an easy sell to sway me off the spec.
My profession, while unrelated to anything automotive, has taught me never to make a decision without the required due diligence. So that is my motivation. I want the facts so I can arrive at my own conclusion.
