There is no need for baseless fear mongering. DEXRON VI is the correct viscosity fluid for the OP's transmission.With Dex VI you'll have the wrong viscosity fluid in that 4L60E for anywhere from 15K-30K miles. If you want a 7.4 cSt Dex III that
holds up longer go with M1 ATF, DW4 ATF, Amsoil ATF, and a host other full synthetics that don't shear much.
5.8 vs 7.4 cSt starting viscosity is not insignificant. If the 4L60E has been rebuilt with modern parts, the Dex VI is probably fine.