I've just topped my oil up at 14,500 miles. I phoned my dealer up, who told me I needed Fully Synthetic 0W30. My local BP garage sells Castrol oils, so I managed to get my oil from there - 1 litre at £17.50.
Is it my imagination, or does the car sound smoother when you top it up again?
320i Sport - AW, 397 alloys