Hi Lee
That's a bit odd, oil out the diff breather is not something we come across often, but someone mentioned it the other day. Annoyingly, I can't remember any details. Kaaz diffs are pretty popular and all the ones I can think of use a SAE90, 75w-90 or 80w-90 with no issues. How long has the oil been in there for? From what I can remember, the oil changes for Kaaz diffs are pretty frequent, something like 1000 miles if I remember correctly. Perhaps trying a 75w-140 LSD oil will sort it out and the Castrol Syntrax Limited Slip, Gulf Gear LS, Red Line 75w-140 GL5, Fuchs Sintopoid LS, Motul Gear Competition and Millers CRX NT LS are good options.
http://www.opieoils.co.uk/c-674-75w-140-gear-oils.aspx
Cheers
Tim