GM does not recommend Dexron ATF for the power steering system in the 8th generation Impala. Read you vehicle owner's manual - it will tell you the type to use. You can purchase the fluid at any GM dealer, bring your VIN to ensure the correct fluid.
Yeah, you may be able to use transmission fluid for your power steering, but personally, I wouldn't. I would use the fluid that GM recommends.
I'm just not sure why you would purposely use transmission fluid in your power steering system when GM specifies power steering fluid... Just use what the manufacturer recommends. Now maybe if you were "stuck somewhere" and all that you had was transmission fluid, then maybe I'd use it, but otherwise, I just don't see why you would purposely use something other than the recommended fluid type.
Maybe you can use transmission fluid without issues - but why risk it?
Use power steering fluid that is GM approved.
ATF "back in the day" but, not so sure that's such a good idea now.
I don't think it's a seal issue but more of whatever the shaft steel and such is made of.