hi got a small leak comming from gear box cant get a streight answer from bmw on amount and type my gear box takes one said 9 liters one said 2 liters etc its got the blue lable on it, so question is what type of gearbox oil and does it have to be bmws type i was gonna get 75w80 norm gearbox oil if its ok to use ?? so what type and how many liters does my z4 2.0i manual 2005 take??? 

been searching for weeks on all forums and this one too cant find anything on the man, 2.0 anywere , phoned 6 different bmws to get different answers and quantities etc then phoned the company that build the box they didnt know eather and as for oil they just said there oil and bmws oil is the same stuff, just a basic 75w,80 except they supply it 11 quid a bottle n bmw is 27 quid i think, all i want is a answer from some one who knows for a fact, the amount of oil i need to do it ?
may as well asked a brick wall