I found a thread about the problem:
Fuel in oil - Peugeot-klub.pl (Google Translate)
Like you wrote, fuel in oil is caused by frequent DPF burnouts. Supposedly what happens is that during the burnout there is an extra squirt of fuel at the end of each combustion cycle to raise the temperature of the exhaust, but often this fuel is not fully burned (especially if the engine is turned off before burnout is complete) and doesn't get expelled with the exhaust. The left over fuel drips down the cylinder walls into the crankcase (or gets sqeezed in there by the high compression pressure in the cylinder). The first thing to get effected by the thinned oil is the turbine. DPF regeneration causing fuel to mix with oil used to be a trademark of VAG (VolksWagen and friends*), but PSA (Citroën+Peugeot*) is trying hard to catch up.
Replacing o-rings or other seals won't help - you have to fix the frequent burnout problem. Pug owners claim that if the regeneration cycles happen every 100km or less, then the DPF must be replaced. Though it is possible that one of the pressure sensors is malfunctioning (there are two presssure sensors in the exhaust: based on pressure before and after the DPF the ECU decides whether it is clogged*). A faulty sensor should show up on specialized diagnostic instruments, like Peugeot Planet. If you've ruled that out then maybe your ECU needs to be reprogrammed?
BTW, I not sure your Volvo does this, but in Pugs the rear window and mirror heaters turn off during regeneration to increase the load on the engine. I'm amazed at the milage you're getting despite all these problems
* - I'm sure you know this, just adding info for any lurkers
