Someone (perhaps you) tried to tell me this before. I am sorry but it is nonsense. Check out
this from the makers - myth I am afraid. Still don't believe it, check out
the MSDS. Section 3 shows the ingredients - aliphatic hydrocarbon, petroleum base oil and LVP aliphatic hydrocarbon make up about 90% of the product. These are all derived from petroleum distillation - nothing to do with fish!
Nothing dangerous, specifically, but what I stated above is correct. It contains lubricating oils which are non-volatile. They do not evaporate away. They stick, they persist and that is part of the reason they work as lubricants (and a large part of the reason why gasoline is useless for long term lubricating - it DOES evaporate so can't lubricate after it is gone). WD40 is
designed to leave this residue and it is
designed to be water resistant. Yeah, it will remove tar/glue etc. because it has all those hydrocarbon solvents but it will also leave behind that residue. That needs really thoroughly washed off. As a quick job, that might be fine but anyone who is justified calling themselves a detailer should be using a solvent based degreaser which contains emulsifiers such that it will need only a rinse, not a thorough wash. In actual practice, you would be better using mineral spirits than this, at least mineral spirits will evaporate away if you leave it long enough.
For reference, my company formulates, manufactures and sells chemical products.