I do believe BGH do recommend comma oil. You can call erm to confirm.
That would be EP80W-90 GL5 for the Diff and EP80W-90 GL4 for the gearbox link attached below. Hope this helps
http://www.commaoil.com/productsguide/view/6/184
http://www.commaoil.com/productsguide/view/6/183
If you are using MT75 there is an oil for that as well, if you check the list of products on the websites you would find it. Also might be a good ideal to speak to the company.
hopes this helps
Regards,
Femi