Per OwnersManual, PowerSteering uses same MerconV Fluid that should be in your 5r55e Transmission;
nonsense to buy overpriced seperate bottle with unknown contents labeled "Power Steering Fluid".
Tight lids on bottles keeps moisture+dirt out, unless you're in an equatorial steam bath climate.
Every 6mths, got a routine of HandPumping ~1/2qt from PS reservoir then refill with fresh MerconV;
also HandPump ~1qt of old fluid from TrannyDipStickTube & refill with fresh MerconV,
along with the usual semi-annual EngineOil+Filter change.