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I see a lot of these videos on YouTube of folks pouring in several liters of automatic transmission fluid into their transmissions as they collect the increasingly cleaner and cleaner fluid that displaces the old. They go some like 5 rounds till all the old stuff is purged as evidenced by the purity of the color coming out of the radiator or some other drain port.

I can see the logic of having unadulterated fluid in the transmission, but I'm curious what exactly is in a tranny that's so sensitive to any colloidal debris remaining in the fluid. Is it that abrasive debris is attempting to be fully removed or that the fluid has lost some of its lubricating properties or some other desirable physical properties over time?

Basically why isn't something like an oil change (drain and fill once) appropriate for transmission fluid?

Here's some examples I gathered of single pass vs multiple pass fluid changes.

Single Pass

Volvo S60 Transmission Fluid Replacement - Smooth Shifts! (S40, S60, S80, V70, XC70) Total of just under 4 liters drained from the tranny.

Multiple Pass

DIY Volvo automatic gearbox flush AW55-50, MANNOL 3309 with oil comparision Total of 18 liters flushed from the tranny. series of sequentially passed transmission fluid

Volvo Transmission oil change Aisin TF-80SC. 6 speed Auto. Total of 12 liters flushed from the tranny. comparison of 3 sequential fluid displacements

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    If the fluid it burnt, then yes it is best to do a 100 percent fluid change.
    – Moab
    Mar 31, 2019 at 19:05
  • Moab, performing a 100% change of the fluid if it is burnt may cause the transmission to slip and perform worse than before.
    – Techlord
    May 22, 2019 at 21:26

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I read the description for the single pass video I linked above and they state the following...

Clean transmission fluid is essential to the operation of your Volvo S60 transmission. The fluid over time can break down and become contaminated with clutch material and damage other parts on your Volvo. Also, the detergent can lose its cleaning properties which can cause shift solenoids to malfunction due to carbon buildup. The transmission fluid in your P2 Chassis Volvo should be changed every 52,500k miles and is fairly simple process taking no longer than a half hour to complete the drain and fill service. While a complete flush is possible, Volvo doesn't recommend to flush out all of the fluid

The key take away is the last sentence, a flush is not recommended apparently by the manufacturer in this instance. There’s also that abrasive clutch adulterating material mentioned as well as detergent with cleaning properties.

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