I have new 2010 toyota corolla with the same transmission. It only has 3500 miles on it now. The factory fill was Toyota WS. After about 2000 miles I decided to replace the factory fill of WS with Redline D4 which has similar viscosity as T-IV. BTW, Dave at Redline recommended D4 for this transmission too. When I drained factory fill there was a lot of break-in wear particles in it, so I am glad I did it. After 6 drains and fills with Redline D4 I essentially replaced all WS with D4. Transmission shifts the same as WS, but it is now a lot more quiet. With WS I had a slight gear whine between 55 and 60 mph under acceleration, especially when the tranny was up to normal temp. I even took the car to the dealer and complained about this whine. It was very quiet (you really had to list for it), but noticable. I was told by dealer that this whine is normal, but I demanded to drive another brand new corolla and compare. During the test drive I heard the same whine around 55 - 60mph, so I don't know if all U341E do it, but it was the second car with the same noise. Anyway, after replacing WS with D4, all the whine was practically gone and almost unnoticable. The bottom line is, IMHO, you should not worry about having T-IV in you tranny for a while. The same tranny was used in 2005 Toyota Celica and T-IV was specified at that time. I thinik they switched to WS for better fuel economy and longer service intervals, but using T-IV (or Redline D4) will not hurt a thing, in fact, it should have better gear protection. Do you have any gear whine with WS?