it looks like it says "whore" vertically to me.
Some people are super weird about stuff like that.
I never knew who it was, but someone who was employed by the last place I worked at used to write 'FAH' on say a quality sign off, then 'Q' somewhere else. There were random boxes with FAH Q written all over the building, and parts shipped to customers with this message somewhere on them (sometimes he wrote it inside the part where no one would look etc).
Some people write their own name on bathroom walls, or carve stupid little pictures or jokes for others to enjoy.
People are weird occasionally. I would do nothing, clean it off... and never... ever go back to that oil change shop.
At least it wasn't one of the usual "left a quickie lube place, 10 blocks later my engine seized" stories.
that blue stuff is easily removed with a finger nail. its put on the drain plug to make sure someone hasnt tampered with it. they usually mark the oil filter with the initials so they can tell who did the work. is the oil filter black? that might be why he used the removable paint to do that also. if it bothers you that much, use your fingernail or a credit card to scrape it off.
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