As I understand it, there was a 1st generation version that was really aggressive, and it especially caused problems with the automatics. Then the 2nd generation units were made in an auto and manual versions. The auto version can be used on both the auto and manual trannys, but the manual version is more aggressive and can only be used in the manual tranny. That being said, I've also heard that the first generation units were the more aggressive than the current manual version.