A common thing with those, from my experience is valve seals, but thats if it was driven very hard and has some mileage on it. After i changed the valve seals, the oil level is quiet steady. It is a very costly job though if you don't do it yourself.
1990 M5 Euro
You wash your car like it was your firstborn child, you tend to its needs like it was your own body, you protect it like it's your family, then you drive it like you stole it