The reason for this was that the DPF was not actively regenerating properly and the reason for that was most likely that is is full of ash. A forced regen by the dealer can get rid of the soot but, by 80,000 miles, most DPFs are starting to fill up with ash and that prevents them from actively regenerating properly with the result that post-injected diesel to fire off the regen sinks into the sump. You either need a new DPF or need it to be chemically cleaned of ash by Ceramex.