yep, i use castrol too, SAF-XA "full synthetic limited slip rear axle oil". i got mine from autobarn. the diff takes 1.5 litres, so i had to buy 2 1L bottles which cost $25 each.
thats not good. i heard that the alternator regulator is the most likely part to fail. unfortionately it cannot be replaced, you need to get another alternotor, but the electrician will lead you in teh right direction