Friction modifier is used for limited slip differentials. No matter what you buy or who you buy it from, GM, Ford, BMW, it is all made, AFAIK, by a single world supplier, an Italian company by the name of Sturaco. Precisely the same stuff, and only the price changes from supplier to supplier..range of cost is 9 to 40 quid. In Canada I buy my BTR fluid from BMW who uses a very similar axle in their 3000 series. Their fluid comes with the additive.
I stopped trusting oil/fluid companies when they all forgot to tell us when they stopped adding ZDDP. 
Without getting too technical, non-LSDs don't need it..though it certainly won't hurt if you use it in one. As correctly noted, the infamous chatter begins if you forget in a LSD. It also lowers the fluid temperature by 5C+.IIRC, correctly Struaco became a BTR need in 1999 when they went to sintered cones in their axles..but that could merely be an cover for catch-up. BTR LSDs changed in 1999 and.issues were cured. The older BTRs are automatically cured when they are rebuilt as the rebuild kit overs it with the needed upgraded parts. I remember less of such issues with the non-LSD Morgan variant. Brian Gateson or Billy B. would know more.
Of course BTRs, like the Morgan Salisburys, are no longer made. But BTR parts are not a problem I understand By the way, BTR is better known as Dana Corporation or for some, Dana-Spicer [
u]in our case their Australian division.[/u].Morgan decisions like this all have a curious history as in how do you put an heavy Australian pickup axle is in a hyper-light sports car! One would think a better choice would have been the world famous and nearby Quaife. Peter Morgan played with that near the end of the Salisbury era, the 12HA was a Salisbury with a great Quaife diff. But then Morgan management changed.
BTR fluid changes are called for every 50,000 miles. I change mine at 25k. I changed Salisbury fluid every 3000 miles.
Lorne
