could be a bad water pump seal from what I was told by a diesel tech at the dealer. There is a seal on the pump that if it goes bad, it will mix the oil and coolant.
I am assuming you don't have any signs of a head gasket being bad correct? The upper radiator hose doesn't stay hard after a couple of hours does it?
2016 GMC Sierra 2500HD Denali
Extang Blackmax Tonneau
Weathertech floor mats and mud flaps
X-pel clear bra
SOLD 2004 GMC Sierra LB7