I wouldn't do it. ATF doesn't have the high pressure additives that a GL-5 fluid (what the owner's manual recommends) which are needed to protect the front diff from premature failure and high wear. I would also have questions about the chemical properties and compatibility issues with the ATF in the gearbox.

 

sounds like your mechanics are interested in job security... if you want "clear up all the notchiness and help synchro's grab better" there are plenty of fluids out there that will do this for you.