There was a couple of threads where the members said that tiny O-ring in their Fuel SupplyLift Pump was worn and that Oil was being sucked from the Fuel Injection Pump Cam area into the Fuel Supply Pump and mixed with the Fuel.
You might get a Glass Jar and take a Fuel sample and hold it up to the light. One of the members said that it gave the Fuel a Blackish tinge. The reason is becaue the Oil+Fuel mix mostly gets returned to the Fuel Tank.
In the below thread there is a pic of the Fuel Supply/Lift Pump in post #3. The Arrow at the far right points to the tiny O-ring.
95 E300D (606.910) Fuel Lift Pump Rebuild Question