That is what BMW requires. Some manufacturers use atf instead of power steering because of the higher pressures, temperatures and the more "cleaning/detergent" action of the atf compared to the power steering fluid if I'm not mistaken. Imagine what atf goes through inside a transmission and compare it to a power steering rack, which is handling worse? (the transmission) this makes the fluid and the system much more durable than using regular power steering fluid
If I'm not mistaken |