I have had the same problem with one of my DZ 170's, but I'm not sure I've discovered the source. The other DZ170 is always clean (I keep the header attached to each so it could be a header issue still).

I recently removed the oily engine to clean everything up to try to solve this one. I tried taping the OD of the header stinger with silicone tape to tighten the seal (comes with YS Super Mount for nose ring) and replaced the o-ring. I ran the motor in the backyard for about 1 minute and the engine looked completely clean afterwards...

I have not flown yet so I'm not sure that this is the solution.