Pretty sure it's based on distance, but months could be factored in, too. But you should be servicing the car at at least twice the "recommended" distance. Assuming yours is a diesel, you should be changing the engine oil and filter at around 10,000km or six months. And by now you should be looking at using a 10w/40 oil instead of the "recommended" 10w/30. Full synthetic, but spec depending upon whether you have a DPF.
There's heaps of advice on here about when to change other fluids. And don't forget to regularly service the EPB.
2013 D4 expedition equipped
1966 Army workshop trailer
(previously SII 2.25 swb, SIII 2.25 swb & lwb, P38 Vogue, 1993 LSE 3.9V8 then HS2.8)
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