Daughter's car was changed to AMSOIL synthetic ATF as part of the purchase agreement. That was some years ago, and fluid is still pink and good to go.
This was a complete flush, by the way. When she gets about 50K on the ATF, I'll change what comes out of the pan.

I am using Castrol synthetic, I think it made shifting a little smoother (very little difference from Mobil 1 synthetic).
However, my mech said that if the car was designed to run on regular fluid, than I should use the same regular.
Anyway, I feel that Castrol is a little better than Mobil 1.

***If your the fluid is REALLY dirty, I would repeat the flush after 1-2 days because you don't chane fluid in the torque converter during flush. ( I guess, you can flush converter as well if you place tranny in Drive and apply parking brakes, and possibly put something in front of thr tires?)

Good luck
Yuri ( no more CEL)

Son-in-law uses Mobil 1 and took care of shifting problems.

dick

I just use Mobil One ATF for piece of mind. I think routine fluid changes are more important than using a synthetic with the 850 tranny because there is no filter in the system. Change the fluid every 25k miles if your not using synthetic, and maybe every 50k if you use synthetic.

Mike

My shifting became smoother running Mobil1 ATF. I wasn't having problems though.