why?
i have used expensive oils and the cheaper Penrite limslip oil,ive found that the Penrite oil makes the diff's quieter, has a larger load rating (140 as opposed to 90w) and whats more its a 1/4 of the price of more expensive shit!!
if you enjoy blowing your cash on expensive oil cause it helps you sleep at night then go for it but theres nothing wrong with the cheaper alternative.
plus ive been using it for 5 years in all my drift cars, NEVER had diff probs.