The special fluid you speak of is JWS-3309 or Toyota-IV, which is particular to the 5sp transmissions.

Rule of thumb:
5 sp = 3309/T-IV
4 sp = DEX-III


If this car is a 2003 as per the thread settings, it almost certainly has the 5sp trans and thus needs 3309/T-IV fluid.

There are many other companies that specify that their fluid is acceptable for use in place of 3309/T-IV fluid as well. The synthetics companies come to mind, like AMSOIL.

To be fair, I'm pretty convinced that this fluid (3309/T-IV) is essentially the same as DEX-III with lots of friction modifier additive.

As far as I can tell, using regular DEX-III in the 3309 transmission would merely result in harsh clutch lockups, which I guess /could/ harm the transmission if you really rocked it hard. But with easy driving I think it would really just amount to annoying loud jerky shifting with little actual damage.

Also, there are companies that sell the friction modifier separately and so if someone accidentally filled their trans with DEX-III I really think you could salvage the fluid (and thus all the money) by adding the friction modifiers.



Additionally, 3309/T-IV in a DEX-III transmission would likely cause no ill effects whatsoever.

I have no first hand experience with any of this though.


--
1998 V70 AWD->FWD->AWD Turbo 220k+