I haven't purchased any so I am not sure (my diff was purchased brand new and was already filled), but I believe if you get the OE specified
oil from Holden for your ZF diff it is all in one container (by the litre), I assume it would already have the additive in it, or was designed like that from the get-go. This is what I meant when I said the right
oil. I think it's about $70 / litre.