back in the day when I had a 1999 Trans Am with a T56 tremec, it was known to have cellulose "paper" blocker rings. Everyone said to NOT use true synthetic ATF in the T56 because it would make the paper based blockers fail, and the synchos wouldn't allow the transmission to shift correctly.
I used Red Line D4 ATF anyway, and had 100,000+ miles of trouble free use. In fast, it shifted so well...all my friends changed as well. They never had any troubles either.
Don't buy into the rumor mill, pour in a good true synthetic, and rock on with it.
It's not a rumor mill.
Use the search button.