904 Transmission Fluid Recommendation

Locomotion said
I'm sure there are people that can provide more details, but if I may.....

As transmissions got newer and especially when computer controls started to be used, transmission fluids were designed to work specifically with the new designs, which vary in how hard or soft they shift, the holding/friction properties and any other factors that may apply. Viscosities also vary between transmission fluids. The wrong fluid fluid can affect how a trans shifts and can lead to shorter life, whether it be from shifting/grabbing too hard and breaking parts to slippage, heat and burning up the bands and clutches. Transmission fluids are a very complicated product. I hope there are some trans techs that can provide more info.

It was more of a retorical question to get him to try to disqualify my post & back up his claim. But FWIW here's the basis of my comment.
A few years ago I was called to testify in a lawsuite case as an expert witness & in addition to myself there was a technical rep from a major oil company & he explained in great detail that there is no way using the wrong type ATF in a transmission would, by itself, cause transmission failure. The ATFs all start out with very similar base stock & then friction modifiers are added to them to alter shift characteristics. Being very into transmissions at the time I asked him a lot of questions. He reminded me the ATF provides 2 things, 1st is the necessary hydraulic fluid to apply the clutch pistons & servos etc. & the 2nd was to cool the transmission components through it's lubrication. That's it, period. The clutches & bands grab their corresponding surfaces with their friction surfaces, the fluid acts as a barrier here & actually impedes the grabbing of clutches & bands. Now before everybody screams, we all know some slippage in a transmission is unavoidable as the clutches & bands apply, but the fluid is only to cool & reduce the wear as this happens. It does not cause the surfaces to grab. Once they are engaged they are held in place by the pistons pressure. This is why performance trans builders for years have been enlarging or adding holes to the drums to allow the fluid to escape from the clutch packs faster to provide a faster shift & make the clutches grab better.
The auto makers at some point felt it's necessary to remove any sensation of mechanical operation that the general public feels when driving a car, so they have been trying to eliminate the shift feeling. Also the recommended service intervals have gotten longer so the synthetic fluids make it easier to go longer periods without changes.
People, both pros & amatures, will often blame the ATF for problems after a fluid change when in reality the fluid was changed after neglect & lack of maintenance or to try to solve a problem that was indicative of a more serious underlying problem that revealed itself only after the ATF was changed. I'm not saying there are no benefits to using a synthetic ATF, but in more cases then not with older cars the benefits aren't that noticable.