I had the same issue with my 2008 Chevy 2500 Duramax. This was a well documented issue at the time. During regen fuel is injected into the piston during the exhaust cycle which which pushes unburned fuel through the exhaust system into the DPF where it burns the diesel particles. Some fuel would find its way back into the oil. Many people corrected this issue by deleting the DPF and reprograming the engine.
The problem was corrected in later years by injecting fuel into the manifold during Regen.