While WD40 will remove tar spots, it is a lubricant not a solvent so it should leave a film of oil behind if its doing its job, its probably an insignificant amount and you really may well buff it off but if you spilled an oil based paint on your shirt you would not use WD40 to clean it off as it would leave a oily stain.
A degreaser is the right product to use whether its cheap white sprit/turpentine or costly tin made to do the same job but anything that removes tar will also remove wax.
John k
