Dave.... Everything I've read (mostly on the "BenzWorld" forum) and my experience so far says no you won't hurt anything. This is a "Smart" tranny... It won't let you put it into a gear that will damage it. I use mine all the time for downshifting and it works great. I figure that if I use that feature a bunch now, while I'm still under warranty, and it breaks... They will fix it. But the SLK folks haven't had any problems with the 5 speed. So my guess is this is one very tuff tranny.