I'd use the same sort of solvent you'd use on tar or other such stuff. E.g., AutoInt New Car Prep, Wurth Cleans-All, 3M Adhesive Remover, PrepSol. Or any other tar remover. Note that these will also strip off any LSP, so plan on rewaxing the area.
Years ago Ziebart had a solvent that they gave you in a blue bottle to remove the rustproofing drippings.I was at a Ziebart when they filled them bottles from a 55 gallon drum. The solvent was deoderized mineral spirits, which you can find in any hardware store.
Actually, to my surprise, it doesn't always work as well as one might expect. It *will* emulsify and remove such stuff, but only with a fair bit of dwell-time and agitation. In fact, I use TAW (preferably in conjunction with my steamer) to clean, but *not* completely remove, paraffin-based undercoatings, most recently on the M3. TAW is *so* mild on this stuff, that I wouldn't recommend it if you really want to remove the undercoating (still a great product to have on hand though).
