Cap can't be rotated. But the grease you are seeing is dielectric grease from a dirty rag I used to try to clean up the cap. I didn't want to use any chemicals or cleaners inside the cap, but after seeing the amount of damage it wouldn't have made a difference what I used to clean it with because it was toast anyways, just thought that I could've salvaged it if it was just dirty

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1990 190E 3.0L