You need to address this immediately before you wash out your main bearings or experience a runaway. What is happening is your DPF is likely partially clogged, forcing the truck into frequent or "incomplete" regens. On the 6.4L, fuel is sprayed on the exhaust stroke of the cylinders to heat up the DPF, and when that fuel doesn't burn off, it washes past the piston rings and into the crankcase.
How you can tell if it's the DPF or a leaking injector: Send an oil sample to Blackstone Labs for analysis. If the fuel percentage is over 5%, you are in the danger zone. In the meantime, stop idling the truck so much and make sure you complete every regen cycle once it starts. Most guys eventually solve this permanently by deleting the DPF, which stops the post-injection cycle entirely and saves the engine from fuel dilution.
