I had thought that the D5 "made" oil from blowby that is why you refill by the set level on VIDA , or if completely drained a set amount which looks like it needs oil if you have a dipstick.
On our D5 it doesn't seem to , may be the oil used is better? I don't know.
Anyway it certainly gets manky and should be done.
I'd do it sooner as it's not a big cost to negate having to clean out the pipes to EGR and dpf etc.