Most oil manufacturers have a recommendation page on their web site where you can enter your reg number and they will suggest which of their products to use. Most oil manufacturers either do not make a recommendation for the Mk4 diff, or say something like “refer to technical”. They do make recommendations for other areas of the car so read between the lines.
t might be worth looking on the Castrol website and putting your reg into the system. I’ve recently replaced all the oils in my ND and they even have LSD oil available. Sourced all the oils from Euro Car Parts.