Just make sure that the Castrol oil contains the LSD additive (friction modifier), although you can get that from Demon Tweeks.
Morgan use Morris Lubricants............
http://www.morrislubricantsonline.co.uk/scripts/prodView.asp?idproduct=68&product=Lodexol+XFS+80W%2F140+Fully+Synthetic+Limited+Slip+Gear+Oil
80W/140 is recommended and is fully synthetic rear axle oil for high performance vehicles. Morris Lubricants have already added the friction modifier to their oil so there is no need to buy the modifier separately.
Other rear axle oil that is approved to be fitted to BTR units are:
Castrol SAF-X
Castrol SAF-XA
Mobil SHC 80W-140
Mobil SHC 80W-140ID
BUT - if any of the above are used then they will require the friction modifier to be used along with the oil.
BMW dealers also sell an LSD oil, but I've misplaced the part number.
I'm with Morris - originally used semi-synth plus friction modifier, but now have fully synth as per the link.
I might also add that the light chatter I got when reversing has now disappeared.
