There is no copper on the gears .
In the gearbox, there are bronze bushings in the reverse idler layshaft .
In the diff, there are bronze alloy thrust washers behind the axle gears.
If one oil is good for the diff , it will be ok , in that aspect , for the g/box.
There are some oils that eat the bronze alloy bushings , I had this in my 4spd once , fortunately I checked it just a month later and saw the oil was heavily metallised , sparkly golden!!.
No permanent damage was done .
