Considering the age of it & time you've had the car I'd definitely stick with a Mazda dealer unless they agree for you to get the oil checked/changed elsewhere.
Is this the 1st time you've checked the oil since owning the car ?, if so due to the time/miles covered it's far more likely that it was overfilled when it was serviced rather than the DPF Regen's dumping unburnt diesel into the sump.
An over filled sump can cause issues for an engine & it catalytic converter if it has one, but oil that's been contaminated by fuel has of course been diluted somewhat it's no longer doing it's job properly and should, along with the filter, be changed ASAP.
FYI all of the diesels cars I've owned and still been under warranty have had any necessary oil & filter changes needed due to contamination issues have been done without cost to me and so they should. The fact some cars have additional (how contaminated is it) marks on the dipstick proves it's a know issue !!!
Good luck