I've monitored my Regen cycle many times and never seen above 1500 but did stay above 1100 the whole time.. If you are hitting 1800 you better start watching your pistons and worry about the turbo seals laterCould also be a turbo oil seal (cooked from 1800 degree regens) or turbo oil drain problem. Do you have a CCV mod by chance??? Some of those raise crankcase pressure enough to cause oil to not drain out of the turbos fast enough (turbos are pressurized oil feed, but gravity drained).
It really depends on what fluid (if any) you are losing. Losing coolant...probably an EGR cooler , maybe a head gasket. Losing oil.....Probably something with the turbos. Not losing any fluid....Probably moisture buildup in the CAC or a regen (if you still have a DPF)