I first learned about this years ago when I bought an Auburn unit. Turns out you can't use synthetic in that either. It ends up making all kinds of grinding type noise when you turn and works more like an open differential than a limited slip. The Eaton and Auburn units are designed with similar materials. Neither can be used with synthetic. Eaton actually changed the clutch material for the Gov-Lock to be able to use synthetic. Something like a carbon metalic type clutch material.

ditto on the previous posts...no synthetic w/ the Eaton...
stock does use synthetic...there's even a tab right by the fill hole that says so...