Hi Mike & Mike
The seal (washer / oil sling) that sometimes fails is at the bottom / around the rotor’s shaft. I’ve seen some cracked pump assemblies as well, most likely due to overheating / operating in an empty well because of the sump pump switch malfunctioning.
As Mike D. mentioned, manual cleaning would be the best choice. There are various oil absorbents (from water) you can use, just search for “oil absorbents” in Google. Hopefully there are no significant deposits inside the drain tile. Ventilation of the area will help as well.
I just hope those sump pump well oil issues aren’t related to any underground oil tanks, I’ve had a couple of such incidents during my home inspections, where an old, underground oil tank started leaking and contaminated drain tile system.