You can use the Morris oil but it's about £90-00 for 3 litres from what I was told, I'm using the castrol B373 as well and changing every 6 months, I've done about 1000 rd miles and the rest at events and the Diff and box have been fine I'll be dropping the oil this month to see what swarf is in there, just to say this oil is not for rd use and will have a short life span.
Regards Russ.............