Mine gets done every 12.5k, for several reasons, in order of priority:
1. That’s an oil change every 3.5months and I really cba to lie under the car any more frequently
2. The D5 lump is known to do starship mileage on the standard servicing, so I’m really not worried.
3. A’s we live in a pretty temperate climate, it’s not like the oil is used in adverse conditions (-25 in Sweden or +40 in Spain for example) so the oil isn’t going to be performing on the limits of its spec.
4. That’s what Volvo recommend.
If you are going to go DIY, I can link you an eBay seller that I’ve had good experiences with. Genuine Volvo stuff (sealed, holograms on etc) at very good prices.
When all’s said and done I don’t think there are any meaningful studies on how oil degrades over mileage and whether this degradation has any measurable impact on engine life. If there were, in sure someone would have linked them by now. As far as I can tell, there just seems to be a consensus that “x miles/months is too long for oil to be an engine”. So, without any empirical evidence one way or the other, just change it as often as gives you peace of mind. Because that’s all you can definitively say you’re buying.