I have always used(with great success) Mobil 1 ATF as P/S Fluid in my Toyota's!
Well, in the last 2 yrs, I have replaced(on our '01 RX-300), the P/S return line(July 2010) and the High Pressure P/S line(July 2011) and in both instances when replacing the P/S fluid, I have used Castrol Import Multi Vehicle ATF in the P/S system.
Jump to NOV 2011 and to date, we started to have sticky/hard/lack of steering return in the power steering. I have never susspected the IMV ATF as the cause of this sticky/hard/lack of steering return but, I am now thinking about it! Although it may just be the P/S Pump, IDK yet.
While we're driving, the steering become more normal but, only while driving. But, when the engine is first started, the steering is "Catchy" with no steering return on it's own until it's warmed up.
I am going to Drain and Refill with Mobil 1 ATF before I delve futher into the steering system. I also use Mobil 1 ATF as the P/S fluid in my '04 Altima 2.5 steering system...Great stuff!
The reason I brought up this issue is because, some years ago on an older Honda Accord, I used an off brand of P/S fluid in the Honda P/S system and I had the "EXACT" same experience as I am having today. When I replaced the fluid with Honda Genuine P/S Fluid(or even BARDAHL P/S Fluid for Honda's), the steering slowly returned back to normal.
So, Mobil 1 is what I am going to try 1st in my Lexus as I have always had good success with it.
Also, I don't believe that when using the WRONG fluid in any system, that problems occure overnight. They may take time as in my instance(1-1.5 yrs).