What would be a good service interval for the diesel model?

I bought a 12/2008 3.0d in mid 2009 with 10Kms travelled.

Had complimentary first service at 20Kms at a BMW dealer, they said I should do a oil service every 6 months or 8000km.

So can we rely on the service indicator?

Recently had an oil leak, sent to a local specialist to avoid rip off dealer cost. They have to replace the PCV value then finally i have to replace the intake manifold which cost a fortune.

The local specialist also suggest more frequent service interval is required for diesel especially more city travel.