I'm getting ready to change my trans filter and fluid and was thinking about switching over to a synthetic fluid. Since you probably can't get every bit of the regular fluid out, with the trans cooler, etc. Would it hurt for what little bit is left in the system to mix with the syn fluid?

 

It'll be ok. It's impossible to get every bit of motor oil out too but when switching to synthetic it's all good so you'll be fine.
-Rippin

 

How about the flushing service people who stick a line in one end and another line in the other and expel the old fluid and suck in new?