the more fm, the less the plates inside the lsd grip (so less noise)... so less locking of course!
it also is aggressive to the oil seal.
so i would only use the fm oil if you really need it and dont want to have noises.
for bmw it is easier and more profitable to sell the fm booster oil and dont have to argue with "customer noises".
if you dont have noise issues or dont mind it, stay with the normal syntrax limited slip for better locking performance and healthy seals!
but when talking about locking performance... if you want a proper performance diff, you will need to swap to a clutch-type lsd anyway... vlsd sucks ;-)