ANTSBM

Can anyone please recommend an engine oil for a 335d m sport coupe 2008 23000miles,just checked oil and it seems dirty,computer says no need to change for another 14000miles im not so sure according to the service book the oil was changed at 20000miles its first and only service!! Had many cars before never a diesel,what does anyone think,why does the oil look so old? Just out of curiosity hom much oil does a 335d hold.Any advise much appreciated,many thanks a bmw ownersclub newbie.