Originally Posted by
Sean Gordon
I would only flush the shifter internals with solvent if they stopped working, IE not letting any cable out. This is a last resort fix for a worn-out STI. You'll never be able to get lubricant back in there after you wash it out. I've had to do this a few times and it usually frees up the mechanism for a little while before complete failure occurs a week or two later. (this is on a cross bike though).
WD 40 will not do any harm to shifters. My 1998 Dura-Ace shifters nearly quit after 4 years. WD 40 totally restored them to good shifting performance and they are working perfectly today. I've known of many cases of WD 40 restoring good performance to other STI shifters.