There has never been an exact answer only guesses. DPF's have been used for about 30 years in both autos and manuals of different makes but even Nissan dealers give different answers as to why Nissan chose the path they did.
There is an explanation of why DPF oil is different somewhere on this forum, no doubt in one of the 150000 DPF related threads, but I can't remember it so I wont try to quote it. If you do a Google search for DPF and read up about what it is, how it works (why it;s not as scarey as many make out) and why it's there you'll probably find the answer....and you had nothing better to do this afternoon.