look at your rear axle, u should be able to see a big drain plug in the centre of the axle. mine is 318i and i already change my axle oil 2 time @ 60,000km interval, it consume about 1.1 litre, its relatively easy and fast to change just like your engine oil. the viscosity of the axle oil is normally 75w90 and cost around RM 30-60 per litre depending on brands