Depends on what the friction modifier does. If it reduces friction, then very few diffs will need it, since Amsoil series 2K gear lube is so slippery. Only diffs with the clutches set on the tight end of the spec *might* need the additive. With dino lubes, the modifier will almost definitely be needed.
The magazine also explicitly states that the Amsoil synth gear lubes don't need the additive.
So, if one doesn't have chatter with Amsoil synth gear lube, one clearly doesn't need the additive. But if, after 10K miles of driving using Amsoil synth gear lube, the diff chatters a bit, then a little modifier might be needed.
Fest3er