Shifting 4th To 5th Problems

I don't have any technical proof, but have always felt that synthetic ATF was "too" slick for these transmissions and a lot of grinding issues occured soon after changing to synthetic ATF. I tend to run conventional ATF in my T-5's.

The synchro blocker rings are made of a specific type of material designed to grab and slow the gear to match the speeds before meshing. I've seemed to feel that the syn ATF is too slick and doesn't allow the material to grab correctly.

I know in the 2001-2004 Mustangs there is a grinding issue that Tremec remedied by replacing the syn ATF with conventional as a bandiaid until a revised TSB gearset was introduced. That was enough reason for me to start recommending conventional ATF in T-5's vs syn.