I dont know about knew car, mine had somewhere around 150K+ when I did it. I don't see how it would hurt a newer car, The seals of an older car seem to be at a greater risk with that trick. Personally, I wouldn't do it again though, the seafoam works great, and is made for that. That is onw of the first little tricks I learned when I started working on my car. I was kinda scared to put the tranny fluid in it, but I went for it. It does work though. And I guess it's strong enough to really get out old junk after 150K. I recomend the seafoam though, it's great.