330i e46 2004 low power steering fluid level ATF

my steering feels heavy, so i looked at the p/steering reservoir and the fluid level is just above the first lower mark. (first out of two level marks)
the reservoir cover showing ATF, so i plan to add / fill ATF.
the color seems to be quite bright pink so is it ok if i just add ATF instead of doing the whole flush?

and what ATF should i use?
i m thinking CASTROL ATF DEX III.
what does bmw recommend using?

pls help me
cheers

Dex III will be fine. Before you add fluid though, elevate the front end, pull the (plastic) splash shield and check for leaks. You said the fluid looked pink- normally it's red. A leak on the suction side of the pump can introduce air into the system creating tiny bubbles and causing the fluid to appear pink (foam). The aerated fluid would also explain the reduced performance of the system. This type of leak can be tough to locate because its not on the pressure side of the pump and won't appear as a big messy fluid leak and it may be the size of a pin hole. Clean everything up good before you check it and pay particular attention to the hose and clamps connecting the reservoir to the pump. This would be the ideal time for a fluid flush and change-

thanks for the reply
so the pink color is no good right?
for the meantime, is it ok to add new atf on top of the remaining old atf in reservoir?

Yes.

could it be power steering pump failure?
pls let me know symptoms of the pump failure.

Pump failure would make for really hard steering.
The shaft shears off from the pump internally. To test for this, remove the belt, and see if you can grab the pulley and pull the shaft out of the housing.