Im changing my differential fluid with Amsoil synthetic and I have a VLSD

Should I also use their Slip Lock additive?

 

VSLD are a sealed module.......not exposed to diff lube other the the non functioning outside case of module. The friction modifying additive won't hurt anything however.

Now a cooler diff lube [less friction......even better an external diff lube cooler] will extend the life of the VSLD module.