While it did not seem to make any difference in my engine it worked great in my transmission. I had changed the fluid and filter in the transmission about a month before adding the Auto-Rx. I had the Auto-Rx in for 1500 miles(slightly longer then application instructions). Today I double drained and refilled the transmission. I drained 2 gallons of fluid out refilled and ran the car for about 2 minutes. I shifted through the gear with my foot on the brake. I then drained another two gallons out. I droped the pan back flushed the almost new filter with about 1 gallon of mild light bodied distilates. I cleaned the pan and reinstalled the filter and pan. I refilled the transmission with another two gallons of fluid. THe 4 gallons that came out today while not oxidized where a very dark red/black color. If you have ever seen a bing chery that was the color.The fluid smelled good but looked dirty. You could smell the Auto-Rx in it. If it was any darker it would have been black. In the past 2 months I have passed 6 gallons of trany fluid through it! It is shifting great! I can only hope that the second treatment of Auto-Rx will work so well on my engine!