I'm not sure it would cause his low oil pressure directly but excessive carbon/sludge deposits could have caused excessive wear, I think his turbo would have failed before any oil pressure issues arose though.
If you have one of these HDi engines it's always best to just change the injector seals out as a service item, apparently number 3 is usually the one that fails.
The Volvo lads say that the fault is not so epidemic as it is in the PSA/Ford cars, wonder what oil Volvo are using and whether that has anything to do with it.