What trans fluid?

[AlanS]

My 2001 S40's tranny is not doing so well...some slippage. I took it to a local tranny mech...he said it appeared "Fine"...no codes, no issues he could tell...however, it does act up when I am driving! The most that I feel I can do as a DIY is to change the fluid. What fluid should I use? I don't want to cause any further harm.

[jimmy57]

2001 uses the 5 speed auto trans requiring the 3309 fluid also known as Toyota T-IV.
The 5 speed has issues that start with valve body and can result in burnt clutches due to low fluid pressure.
Flush may not address the problem but is the place to start.
Slips in which gear?
Any strange downshifts as you slow to a stop?

If it has 2-3 shift flare there is a servo cover that can address that concern.

[AlanS]

2001 uses the 5 speed auto trans requiring the 3309 fluid also known as Toyota T-IV.
The 5 speed has issues that start with valve body and can result in burnt clutches due to low fluid pressure.
Flush may not address the problem but is the place to start.
Slips in which gear?
Any strange downshifts as you slow to a stop?

If it has 2-3 shift flare there is a servo cover that can address that concern.

People have said to use Mobil 1 synthetic...is this compatible?

[jimmy57]

I picked up some Mobil 1 at an auto store and it did not list compatibility for 3309 or T-IV. I found by looking at labelling that several of the fluids reduced the applications they worked in.
I know some people on here have used it but the AW 55-50 gearbox does some funky stuff with reducing fluid pressure on some applied clutches during shifts to soften the release and application that occurs on other elements in gearbox. The slipping of the clutches and the slipping lockup torque converter strategies require some additives that may be omitted from fluids that try to be "suitable" for a lot of different applications as opposed to meeting the requirements of specific fluid types.