As per my axles thread on the subject, I checked with Vx as to what goes in the limited slip differentials found on some Fronteras.
Standard gear oil (motor factor will be cheaper)
Oil additive, part 90004033 (one tube per differential at oil change)
As at May 2012, this cost me £5.10 including VAT at my dealer, just took a couple of days from ordering to arrivall as it's not a stock item.