My previous f-bodies were autos and a buddy told me the other day that I should not put synthetic in a M6.

I had never heard this, what are the reasons/negatives?

 

i use royal purple

 

I use Royal Purple as well. I think the T56's stock say to use ATF in them. Royal Purple makes a special transmission fluid specifically for manuals that require auto fluid.

 

All manual and auto transmissions use ATF. There is no Manual Transmission Fluid. And synthetic is just fine for your T56. I used Mobil 1 in mine. Your friend might have been thinking about gear oil, as gear oil will destroy the blocker rings in the synchros of the tranny.