2021 Prius Prime WS change

IMHO, Your changing the ATF to prolong the life of the car/transmission - cheap maintenance insurance. Why introduce an unknown variable into the equation? While the Valvoline ATF claims it compatible - it isn't substantially cheaper than the OEM ATF. This is an infrequent fluid change that your going to do every ~100K miles. I would go with what I know works, Toyota ATF WS, if you didn't get the hint! You can dump regular Dexron or Mercon ATF in and that'll probably work for a while. Just because your transmission doesn't blow-up right away, doesn't mean that ATF isn't doing incremental damage to the transmission. Remember why your changing the ATF in the first place? Toyota states that the ATF is lifetime, so changing it every 100K to dump out sediment and filings is a very good preventive measure. I wouldn't cheap-out, if your planning to drive that car till the wheels fall off.

Bottom line; It's your car - put in whatever you feel comfortable with......

 

I'd second to use Toyota ATF WS; I mean, why not?

Second gen had a magnet on drain bolt, third gen doesn't, probably 4th gen doesn't either.

There's some 3rd gen specific transaxle fluid change info in a link in my signature (on a phone turn it landscape to see signatures), mostly relevant for 4th gen.

Attached is from Repair Manual

 

Webber video shows there is a magnet at the bottom of the Gen4 tranny case.