I have a 2010 Mazda3, automatic transmission, with 17k miles on it. Couple weeks ago I had the oil changed and tires rotated, and the tech said that the transmission fluid was dark brown and needs replaced. I have also noticed that the car seems more sluggish and is taking longer to switch gears. Does this sound like an issue that I should have checked out by the dealer under warranty, or should I just have my local shop replace the trans fluid? Has anyone else noticed this issue so early in their driving experience?