just purchased a used low mile (126k) 2010 Corolla, went for an oil change and the trans oil was very dark / black... took to Toyota to have transmission fluid change @ $270 .. check fluid level about 2 weeks later and notice remnants of dark fluid still on dip stick, will take in for another fluid flush again in a few months .. very disappointing for a Toyota and especially a Corolla their flagship model. Now I find this is an on going issue with this model. I will now have to constantly check fluid for "color" and most likely have to change fluid 1-2 times a year!!

Toyota is recommending the WS grade for transmission fluid on all new cars. Either the design of the 2010 Corolla is very poor and the transmission has to work harder or the fluid is very thin and poor. I noticed the WS transmission fluid is thinner than the Mercon III. I supposed it is for easier shifting. The oil got dark and dirty so quickly and I can think of my corolla will have to change the transmission fluid when it is only about 50K miles. I am just finding more and more problems with the 2010 corolla. This is in addition to no accelerations on the car and also self-acceleration on the car when you try to stop instead.

Update from Jun 3, 2014: Normal driving and the car was making a lot of noise. The transmission fluid was dark, and like burned. I suspect it was due to the hesitation on acceleration, and hard to stop problems. After changed the transmission fluid, the engine noise is gone. The manual has no transmission fluid schedule maintenance at 50K or even 60K other checking the level of fluid.