Another update;
FIW I asked Castrol if their new Transmax 75W140 is suitable for this axle. Their reply in Dutch below, plus a translation.
"De opvolger van Castrol SAF XJ is Castrol Transmax Limited Slip LL 75W-140 en heeft dezelfde eigenschappen, het product bevat de LS additieven en is tevens BMW M en Mercedes AMG goedgekeurd"
"The successor to Castrol SAF XJ is Castrol Transmax Limited Slip LL 75W-140 and has the same properties, the product contains LS additives and is also BMW M and Mercedes AMG approved"
This is also what the Dutch Morgan importer uses.
I have driven the car with this oil and its is all quiet at the rear but I have not got the diff up to "working" temp yet. When the weather improves I will and report.
Regards, J